FeaturedUsersis an interesting one to explain and is from the makers of FriendorFollow. You purchase impressions from them to get your profile shown on the Twitter apps in their network. The more you buy the more your profile shows. This supposedly gets you more followers. To start the process you enter your Twitter username (no password). They then pull your bio information from your Twitter profile to build a banner that would show.
Your banner then shows that you are sponsoring the application that the person is looking at. The entry cost is $10 for 1000 impressions and goes up from there. For $100 they give you an extra 500 impressions, making that one 10,500. They do provide tracking and statistics for your hits but you have no control over what apps show your sponsorship.
Now, you can get some free impressions and you are relying on someone actually buying a Twitter application. Those are hard to come by with the incredible amount of free tools out there. One last thing, is you can use FeaturedUsers to advertise on your own site, but once again you cannot do anything about it if a profile shows up that you do not agree with sponsoring the application.
So my overall opinion is while this seems to be a valid attempt at monetizing a facet of Twitter, there is a big gap in the control and interaction. As well as a gap in the amount of applications avaialble for purchase out there.