Modern web apps can be developed to provide a super cool experience to fully capable browsers, and an acceptable (although not quite as shiny) experience to less capable browsers. We've been doing this for years with best practices such as progressive enhancement, so let's keep up the good work.
No core guides listed as yet. Contributions appreciated.
No new technologies needed — the Web, and established best practices, have worked like this for a long time!
Link to any useful tools here.
Link other useful information here.