This is becoming one of the most popular websites in the US. It's essentially a free, online classifieds section, but really useful. If you're travelling in the States, check it out, each city has it's own page. Especially good for travelers is the rideshare section. Craigslist is also in several other cities in the world but has not caught on the way it has in the US
This is a bulletin board website for backpackers. It is free to register. I really like this site, it also has a map that shows the current location of all members.
Just what the name says
Has the latest information and warnings for US travellers