Latest entry in the local guides field: Nextstop, which lets users create their own local or global guides—think best beer bars in the world, for example.
Founded by several former Google employees, Nextstop lets users read and build various guides covering anything under the sun, from the the best beer bars in the world (mentioned above) and best beer bars in Massachusetts to hidden spots in San Francisco and Go! Play! Portland! (a fun tour of Portland).
You can search for guides on Nextstop by places, guides, and map, or just focus your search on a specific city. The site was in closed beta until today, which helps explain why a good number of my NYC recommendations included more obvious tourist spots (think Statute of Liberty) and less off-the-beaten-path selections. Of course, if and when more users create their own guides, the site could potentially be a decent resource for interesting things to do in a given city, especially centered around a specific theme.