The idea behind Billshrink.com is pretty simple: you let them review your cellphone bills for the previous months, and they recommend a new, cheaper plan based on your phone usage. The site also features a neat Google maps mashup that shows you what reception levels different carriers have in different neighborhoods.
All you have to do is give them the login and password to your carrier of choice. (It works just like the financial site Mint.com does and appears to be secure.) Then they'll spit out a bunch of plans to choose from. I tried it and found that if I switch from my current Sprint plan to a new T-Mobile one, I could save about $60/year. (Which I'm not gonna do, considering I just bought an out-of-contract Blackberry Pearl that only works with Sprint, but if the savings were a little bit higher I'd be tempted.)
Right now, Billshrink only works with the major four carriers, but they tell Consumerist they're going to be updating their algorithms to include more carriers, and take into account that many folks are now using their cellphones as landlines via [email protected] setups.