A few months ago, Twitter added a Lists feature. I've used it a bit to categories the twits (my term for Twitter users) I follow, but only recently have I really started to use the feature.
The problem is that I've started following more and more people, and the tweets of the ones I know best were getting lost in my home page timeline. So I setup a private list called "favs" which allows me to add the ones I really didn't want to miss. If I click on this list, it shows me only the tweets of the twits on the list.
I've also got lists for family, twits in my metro area, close friends, fellow musicians, tweetup twits, etc. It's a great feature.
Now I'm gonna need to see which of the iPhone Twitter apps support the lists feature. I'm using mostly Twitterrific right now, but I don't think lists are supported yet. Probably just a matter of time.
So if you're not using the lists feature yet, you're missing out. And if you're not using Twitter yet, well.. what are you waiting for?