Bottom line: Formulists is an awesome webapp that autopopulates lists based on user-defined criteria.

For those of you jumping on the Twitter bandwagon — Twitter's "Lists" feature seems to be really underutilized.  Most people don't even realize that you can add someone to a "list" without actually following them.  This makes it almost like having multiple Twitter accounts (which is another viable option) in that these users won't show up in your primary timeline, but at any time you can go to your lists, each of which is like its own timeline.

One example of how this is useful is for using Twitter as a news tool.  I follow a number of medicine-relevant twitter accounts like @NEJM, @theNCI, @NPRHealth, and several others.  If I didn't want all of their tweets to crowd my main timeline, I could just add them all to a list called "Medical News," which I could then check whenever I wanted.
Your lists can be public or private.  Public lists can then be followed by others, private lists are... private.  For example, I have a private list called "Matters," which I've manually populated with "real" people that I actually know in person.  It's nice to scan this list on occasion and have all their tweets in one place.  I have others for iPhone and Mac developers that I like, since their Twitter feeds are frequently the fastest way to get support and/or news on updates.
@Formulists is a cool free WebApp that can automatically populate lists for you based on a number of criteria.  These lists then show up in your regular Twitter lists.  Some of their handy auto lists help you find new tweeters to follow by listing people that share common interests or follow the same people you follow.  Other lists help filter people you follow by location, by number of followers, by whether they follow you back — all kinds of stuff.  One of the handier lists I have is just a list of accounts that I haven't listed.  I can check this every so often and quickly categorize the accounts I follow to keep everything tidy.  Another filters people I follow by location, so I can check out what trouble is brewing locally.
Anyone else find any good use for Twitter lists?  To be honest, I find it a feature with lots of potential that I persistently underutilize.