It's an increasingly common Twitter experience — you mention a company in passing and suddenly find that you're being followed by its Twitter marketing robot. Nest Unclutterer helps keep your follower list free of those kinds of distractions.
The free Web-based service from the Unclutterer blog automatically removes "suspect" followers, including accounts with suspiciously large numbers of follows (which you can customise), and allows you to stop following accounts that have been inactive for a long period. You can review the service's selections and whitelist any actual friends (or interesting strangers) before blocking commences.
If you've been following people indiscriminately (perhaps because of a misguided belief that it's bad manners not to follow someone who is following you), Nest Unclutterer is a useful way to get some order back into your Twitter universe. Thanks Di!