If you're looking to find a place to crash in a foreign city and have tried solutions like Craigslist or AirBed & Breakfast, you've got to check out Roomorama. It's a peer-to-peer rental community that lets visitors find a cheap, low-key place to stay, and gives renters with some extra space a chance to make some cash.
Like AirBed & Breakfast, people with some extra space can put their place up on the market. If someone's coming into town during the dates you set as being available, they'll have the option to book it. Once you approve the booking, the payment goes through Roomorama's system (which uses PayPal) and the traveler gets a confirmation.
As a traveler you can sort out listings by all kinds of factors, but my favorite is the simple matrix of amenities. You can click to highlight the things you want, like Wi-Fi, parking, laundry, and the all important hot tub. It'll filter the results in real time with every click, and if there's not something that matches up with what you're looking for you can opt to make it a request. If someone's been on the fence about listing their place they can then claim your request with their offering. If an agreement is met you can book it on the spot.
Roomorama is currently limited to New York City with other cities to come. In the meantime, you can list and request rooms in different cities using the aforementioned shout-out system that does the matching for you.