Back rests - see the DIY & Repairs page
Ballasting sea kayaks / altering seats to improve stability - discusion. See this excellent article on using dive weights to add ballast.
Boat packing / hints & tips - discussion. Sea Kayaker magazine has a .pdf with a packing diagram.
Boat reviews can be found on the US Paddling.net site - very US orientated of course, but most of the popular British boats are included. There are also some boat reviews here on UKSKGB's Reviews page.
Choosing sea boats - discussion here and here that includes thoughts on boat choice & design. This discussion on "day boat / expedition boat" choice includes thoughts on boat size relative to paddler size and gear. This discussion covers plastic v f/glass as well as Seayak, Romany Explorer, Nordkapp & Capella. See this discussion on boats for "the larger paddler". And also this discussion on boats for "the smaller paddler" and children, as does this discussion. This discussiondeals with the popular UK boats and paddler perception. This discussion gives some ideas for choosing a double sea kayak, as does this one.
Choosing sea boats - here's a massive directory of just about every sea boat available in Europe. Although a bit out of date now (1998) this article on choosing a sea kayak by Altlantic Kayak Tours is worth a look. This article from One Ocean Kayaks gives a lot of insight into the factors involved in making a choice and includes an excellent insight into primary and secondary stability. So does this article from Seakayak Chesapeake Bay. As does this one from Guillemot Kayaks. Canoe and Kayak UK reviewed some popular plastic sea boats. Pam's blog "Kayak across the water" includes this excellent table with statistics on a wide range of (mainly British) boats.
Choosing a surf kayak - discussion. See also here and here
Diolen or kevlar? - discussion
Folding Boats - see this discussion for thoughts on Klepper, Folbot, Pouchboats and others. Folding Kayaks.org is a forum dedicated to folders and aligns to the interesting site run by Michael Edelman. Tom Yost can teach you everything you ever wanted to know about DIY folding boats. Wayland Folding Kayaks offer, apart from their own range, what seems to be a comprehensive range of accessories, replacement parts and information. This discussion is about a variety of folding boats.
Getting started - first boat, general advice etc - Discussions here and here.
Greenland Kayaks - interesting .pdf with a lot of historical background. Thanks to Erling for the original .pdf. Duncan Winning kindly provided an article on the history of the Greenland kayak, incorporating the "family tree" of some 43 different craft. There is also a very detailed history of the Greenland Kayak's importance to UK sea kayaking entitled "It's Inuit int.it?". A version of this article also appears in Sea Kayaker magazine. The History page has a number of links to useful historical background.
How a fiberglass boat is built - interesting article from Kayarchy.co.uk on how these boats are built.
Hydrodynamics - what makes for a "fast" boat?
Hydrofoil Kayak - "The Flyak" is still under development.
Insurance for sea kayaks - discussion
Outfitting and modifying sea kayaks - see the DIY, Repairs & Modifications page
Plastic sea kayaks - discussion
Plastic sea kayaks - group test of the Point65 Crunch, Valley Avocet and Aquanaut and P&H Capella 166.
Plastic surf kayaks - discussion here and here
Repairs - see the DIY, Repairs & Modifications page
Sails - see the Equipment page.
Seats - see the DIY & Repairs page
Skegs - see the Technique Page for details on how to use them and the Repairs & Modifications page for how to fix them.
Take-apart / multi-part kayaks - excellent website by Marcus Demuth on his experiences. For DIY'ing the job, see the Repairs & DIY Pages.