This bookmarklet uses brainwallet and bitcoinjs-lib to perform client side message verification. A user running a group buy spends a lot of time copy pasting across windows to verify signatures and asked for something to make it easier.
If you go here there's a little page to drag the bookmarklet above to the address bar.
http://bl.ocks.org/david4096/raw/5883334/Directions:
1) Copy message to verify.
2) Select signature.
3) Click bookmarklet, paste message.
4) Output is written to alert and javascript console.
This has been tested in Chrome.