Informal testing sessions on the street, in cafe's (also called guerrilla testing, ghetto testing,...) are a great and inexpensive way to get valuable feedback from potential customers of your software. I wrote about the topic before here.

One of my favourite websites last.fm is currently beta testing their next version and guess what they use guerilla testing to do so. Fortunately they also spread the word and write about it. So hop over to their blog and learn how they organize their testing sessions and how it works out for them.