Backrests - see "Seats" below.
Building sea kayaks - see the Sea Kayaks page for links to a variety of "self build" articles and sites on everything from skin-on-frame to wooden boats.
Compass - illuminating a deck mounted compass - discussion
Cleats - see this discussion for thoughts on fitting cleats and fairleads. This discussion deals specifically with fittings on plastic boats. Discussion with advice on waterproofing and fitting reinforcement plates.
"Finishing a North Shore Calypso" - excellent illustrated article by "Erling" on completing a "bare hull" Calypso. Relevant to other f/glass work, fitting seats, decklines, footrails etc.
Fittings and bits and pieces - Sealect Designs has a lot of useful stuff (System X no longer import Sealect Designs, contact them direct).
Foam - closed cell foam for outfitting, making backrest and footrests etc - discussion with suggested suppliers.
Gel coat repairs - see "Repairing f/glass" below.
Hatches - discussion on making VCP hatches fit, easing removal etc. Leaking hatches are discussed here. See this discussion for info on sourcing KajakSport hatches. KajakSport also make replacement covers for some Valley hatches. SeaLect Designs make alternatives to Valley and KajakSport covers.
Keel strip - good article from Atlantic Kayak Tours on fitting / replacing a keel strip. See also this discussion and this discussion. As an alternative to the usual glass or kevlar keel strip, the Keel Eazy is worth considering - available from Reed Chillcheater and also from KariTek - and others no doubt too.
Lap belt for a surf boat - discussion with links to pics and details of how to do it.
Outfitting sea kayaks - article with ideas and suggestions for modifying sea kayaks, installing pumps, tow-lines and various other things. See also Gnarlydognews - for gear tests and superb DIY ideas.
Pumps - fitting an electric pump - this discussion shows a submersible pump installation - this one has a detailed description by Jim Wallis of his installation. This one by JulesT is also very interesting. See also the section on pumps in the article on outfitting sea kayaks. Fitting a footpump in a poly boat can be a little more challenging owing to the fixed bulkhead position, a bulkhead which itself may not be as secure as that in a f/glass boat - this discussion includes a photo from the now defunct Dutch site Zee Uitrusting which offers a possible solution. Foot pumps - repairing the Henderson pump - discussion. Parts for Henderson / Whale pumps are readily available from ASAP Supplies.
Renovating and/or restoring f/glass boats - this discussion details the renovation of an old Nordkapp but has a mass of useful information on the subject. Also worth looking at is this discussion on the renovation of a Sea King, especially notable as the owner had it painted.
Repair kit - article with suggestions for a repair kit.
Repairing boats (UK resources) - Rockpool Kayaks www.rockpoolkayaks.com is recommended for repairs, and Sea Kayaking UK (formally NDK) also seem to offer a generic repair service. Bournemouth Canoes can arrange repairs. The major manufacturers usually offer a repair service for their own boats, certainly Sea Kayaking UK (formally NDK), P&H and Valley Sea Kayaks all do. For repairs in Banbury, Southampton and Dorchester, check this discussion. Sea Kayak Services are based in Weymouth, Dorset and are "dedicated to providing a high quality service to all sea kayakers". Fibreglass Fabrications Ltd, in Paisley, PA3 4AL, have been mentioned as doing a decent job, and Roddy who runs Kayak Bute has had good service from the boatyard at Port Bannantye, as detailed here. Summit to Sea on Anglesey can repair boats.
Repairing boats - See this discussion for a mass of detail, including techniques and suppliers of materials. Also see this discussion. This discussion references how to add fittings and do minor repairs, and suggests the use of epoxy resin. This article from Happy Paddlin' describes how to repair boats made from Carbonlite / Thermoformed Composite Construction (and also Royalex). Kayak Bute have a good illustrated description of how to repair gelcoat on their blog.
Repairing f/glass / kevlar kayaks - very detailed article by Chris Bolton on doing repairs to composite boats.
Repairing f/glass - material sources - CFS Fibreglass Supplies offer a mail order service. East Coast Fibreglass have been recommended by a number of people.
Repairing folding / skin-on-frame kayaks - Folding Kayaks UK offer, apart from their own range, what seems to be a comprehensive range of accessories, replacement parts and information. They offer a variety of replacement hulls as well. Replacement Folding Skins can be made by www.pouchboats.com. The rigid hulls are made from 18oz PVC. This material is used for truck covers and can be found under tarpaulin makers and marquee manufacturers. The decks are 10-12oz proofed canvas (unpainted to be allowed to breath). 12-15oz proofed canvas can also be used for the hull if it is painted with two coats of spirit based top coat, but PVC is stronger, does not need painting and is maintenance free. See the now defunct Yostwerks [on archive.org]. Possible sources of tarp material are www.Tarps.co.uk and The Protective Textile Co Ltd.
Repairing side-seams on f/glass boats - discussion. This discussion also deals with removing an old fibre glass seam. This discussion includes a detailed description of how to repair and strengthen a seam.
Repairing plastic boats - the "tarp repair" offers a possible solution to fixing deep scratches - using a red hot putty knife to melt/press strips from a pvc tarp into them.
Sails for sea kayaks - Laurie Ford [on archive.org] offers some thoughts on designing and fitting sails on his site. The Maatsuyker Kayak Club has this article with some designs. From New Zealand, plans and instructions [on archive.org] for a Polynesian style sail. Douglas Wilcox offers some useful insight into fitting a Flat Earth sail to a kayak. See also the Equipment page for more information on fitting and fettling sailing kit.
Seats & backrests - discussion. Outfitting sea kayak cockpits / seats - discussion here and here. Seats & kayak outfitting - discussion. Seats - lowering the seat in a P&H boat - discussion. Seat replacement solutions - discussion. General discussion on back rests and preventing back pain. This discussion includes pics of a diy back rest made from foam, as does this discussion with other useful comments. This excellent article [on archive.org] offers advice on preventing backpain. See also these articles from Kayak Fit and the Cockpit Outfitting section of the article on Outfitting Sea Kayaks.
Self build - see the Sea Kayaks page for links to a variety of "self build" articles and sites on everything from skin-on-frame to wooden boats.
Skegs - This article on repairing skegs shows how to remove a skeg and also how to fix a variety of common problems, including how to replace the wire and reassemble the system. It also serves as the source for other links, discussions and articles related to making, maintaining and repairing skeg systems. See also this article on modifying the skeg to make the wire easier to replace in the field. Fixing a cable operated skeg - this discussion will help, as will this one. This one will also be of help. notably with regards to Valley boats if the skeg control has rusted. Superb illustrated article by "gnarlydog" on making and fitting a skeg. Kari-Tek produce a fantastic retro-fitable skeg system. Andy Waddington built a wooden skeg system [on archive.organd his article is worth a look.
Spray decks - make your own spray deck, from WhiteWaterSlalom.Net. See also Duane Strosaker's superb DIY spraydeck page [on archive.org] and the Baker Brothers also have a nice description [on archive.org] of how to make a spray deck (no pics though.
Stainless steel fittings, nuts, bolts etc - A2A4 Fittings Ltd are the people to deal with. There are many suppliers on eBay, just search for the bolt type and check the feedback and delivery.
Tape - "helicopter" tape for deck protection, Solas reflective tape, "hook & loop" (Velcro) tape, anti-slip tape etc - eBay is a good source for small quantities. Although I note that some helicopter tape does yellow badly!
Three-part (or two-part) kayaks - "Ceegee" modified a standard Rockpool to make a three-part version - a truly amazing job. These three posts detail the work involved - Initial thoughts - Part 1 and Part 2 of the job.