Startup travel Web sites are launched about as often and optimistically as restaurants, but only the hardiest survive. These four are worth bookmarking. Social-media site Gogobot.com takes recommendations from your friends on Facebook and Twitter and catalogs them into a personalized guidebook for your next trip. iStopOver.com is an easy-to-use site for home and room rentals with reviews of the properties—and hosts. Green.travel, launched by Sustainable Travel International, is a search engine for eco-friendly trips, allowing users to sort by activity, country, and type of lodging. Zozi.com vets and then partners with local outfitters to market adventures—from fire-breathing classes to abalone diving—near ten U.S. cities.