Twitter users have recently begun receiving spam claiming to be an "unfollow app" capable of telling you who has stopped following you on Twitter.
Since this "app" is being advertised via spam, it should of course be treated as suspect.
The spam uses multiple redirects to fool scanners:
First Redirect Destination Analysis: (Clean)
Second Redirect Destination Analysis: (Potentially damaging content per Websense Threatseeker)
At the very least, this is most likely a scam to get social networking impressions. Chances are fairly high, however, that this could be malicious software.
If you receive any spam advertising this (or any other app), report the account to Twitter and they will deal with it accordingly.