What makes a game accessible for you is completely dependant on your own abilities. The following list gives some initial pointers. Until there is a ratings system for accessibility in place, you will likely have to rely upon trial and error to find games to suit. Please feel free to e-mail for help on anything that is unclear.
This list is not conclusive, but should help you track down a good range of accessible games.
Accessible GameBase - Huge list of games.
Audio games - Games for blind gamers.
AbleGamers - Various games.
Deaf Gamers - All kinds of games.
Eye Tracker games - Eye Tracker games.
Head Tracker games - Links and help.
One-switch games - Switch gaming.
UA-games - Universally Accessible games.
UK charity AbilityNet sell the "AccessFUN" memory stick for around £16.99. It is chock full of accessible games and activities.
Sold at cost price, covering research and development.
Amazon - A good on-line source of both old and new games.
Play.com - Great prices for new games in the UK.