Brooke, Lindsey, and I have all been asked more than once how we possibly afforded our Euro-adventure. We had to be pretty frugal. There was a lot of grocery store trips (nothing is cheaper than pasta), food splitting (or "rationing" as we called it), picking and choosing of what "splurge" sites we would pay to see, and just general attempts at being thrifty. However, nothing saved us more money than Couchsurfing.com.
For those of you who haven't heard of this webpage, it's basically what it sounds like, except sometimes you get a couch, sometimes you get a bed, sometimes you get a futon, sometimes you get a floor, sometimes you'll take whatever you can get. Essentially you make a profile and then request to stay in strangers' homes.
Now the appropriate and usual reaction to this idea is "STRANGER DANGER!!" While this is a practical concern, we had a really great and safe experience with the whole thing. Profiles are linked to reviews from other surfers and that's largely how we chose who we stayed with. The people you request to stay with must also approve you, and you can talk with them prior to your visit.
To be fair, there is a decent part of this that is trusting your gut. I am sure that not every Couchsurfer is a safe and hospitable new friend. There are creepers everywhere, but just as well, there are wonderful people everywhere. We tried to be as safe as possible, like I said, trusting other surfers' reviews and especially steering towards people with a large quantity of positive comments.
While I had my first Couchsurfing experience on my trip to England, and Brooke & Lindsey had their's in Florence (while I was off galavanting in Southern Spain), we had our first joint experience in Geneva, Switzerland.