Twitter lists are a great way to organise who you follow, but manually curating the lists can be time consuming. Formulists is a robust tool for auto-creating and managing Twitter lists and finding more people to add to your Twitter network.
Link your Twitter account to Formulists to group "tweeps" into a great many categories. Formulists offers stock lists like top people who mentioned you most and people you mention the most. You can filter people by how often they tweet (e.g. create a list for people who tweet too much), their location or bio, or other criteria. Other lists can help you discover people that share Twitter lists with you, track your new Twitter followers, and a whole lot more.
In short, it's basically a way to turn Twitter into a more powerful networking tool via easier list creation.