Note: After spending a couple hours revising this page, I realized this is exactly the kind of thing del.icio.us is perfect for. Doh! I’ve created http://del.icio.us/maphacks to continue keeping track, let me know if you’ve found some I don’t have up yet (my e-mail address is in my profile).
Some of my favorites (note that many of these do not work in Safari):
– GeoBloggers, (collaborative project)
– Foundcity, bookmarks for the physical world (currently Manhattan only)