How to do accessible forms
WebAIM has a section on creating accessible forms. As they say: "everyone benefits from a well-organized, highly usable form, especially those with cognitive disabilities". They also give useful technical tips on accessible forms controls and on using DreamWeaver and FrontPage to create accessible forms.
Jim Thatcher has a tutorial on accessible forms to meet the requirements of Section 508. Note in particular his point about NOT putting the prompt text within text entry field. We've seen many users make mistakes when the prompt text is in the field: they don't realise that the field needs to be filled in.
The basics of how to achieve accessible forms by Roger Hudson
HTML Dog has a concise summary of how to mark up labels, fieldsets and legends, and option groups
Nick Rigby writes on A List Apart on how to mark up prettier accessible forms
Mert Tol has a great Checklist for better forms that includes tips on visitor support, error handling and form submission as well as accessibility and the most crucial topic of all: minimising visitor effort.
Aaron Cannon has an accessibility checklist reprinted here The accessibility checklist i vowed I'd never write and includes six guidelines for forms.
Book on accessibility
Shawn Lawton Henry has made her book on accessibility Just Ask available online. Her theme is: bring people with disabilities into your project, work with them, just ask. She doesn't specifically deal with forms but her approach certainly works really well for forms.
Background: inspiration and discussion
Interview with Joe Dolson of Accessible Web Design about the importance of accessible forms
Jeremy Keith and Derek Featherstone have a long chat about how to think about AJAX from the point of view of accessibility
Donna Jodhan rails about the problem with PDF content; forms created as PDFs may be difficult or impossible for people who find reading difficult.