One of the fastest is Bitcoin-Brokers which facilitates both the buying and selling simultaneously via cash deposits into major banks such as Bank of America, Wells Fargo, Chase.

There are hundreds of BTC available at any of those banks.

Sellers typically receive between MtGox-MtGox +3%. The net costs to buyers usually runs 2-5%.

Here is a 6700 view thread discussing the service:

https://bitcointalk.org/index.php?topic=237164.0