The Twit Cleaner is a script that runs through those you follow looking for bots and people that only post links. It then gives you a detailed report via direct message (so you must follow them for a bit) and allows you to decide who to unfollow. They use oAuth so have no fear there.
If you follow fewer than 2,000 people the service is totally free. Over that is is only $5 via PayPal, not a bad deal or way for them to pay for the service itself. When you get the report you can hover over any avatar and it will give the reason why they were listed as someone to possibly remove. The bad guys fall into some categories:
The possible categories are:
Dodgy – spam phrases, @ spamming, duplicate links etc
Absent – No updates in a month, or fewer than 10 tweets.
Repetitive – High numbers of duplicate tweets or links
Flooding – So high volume you can’t see anyone else
Non-Responsive – No interaction & those that follow back < 10%
Secretive – No bio/url & shortened profile urls (this is sometimes ok)
From there you are able to decide who gets the unfollow and who doesn’t. You can see a huge sample report here to get an idea of how it looks. Give it some time to run based on the number of people you follow and how busy their requests are (such as when everyone reads this posting).
A great service and worth sending them a couple bucks.