I love this utility because it makes it very easy to switch back and forth between 2 or 3 programs with one or two alt+tab or other shortcut you prefer while at the same time you have 20 other programs running in the background. I find myself often switching between BBedit and Terminal and doing this for hours, then this is a great utility to use. It's in Public Beta, but I have used it for a month without problems.
Every time you alt+tab, a small switching window is displayed which contains the list/icons of all running applications. If you click on one of them you jump to that app; alt+tab+q quits the app.
Shareware, available here.
[Editor's note: I haven't tried this one myself, but it sounds like it's worth a look. I find the new alt-tab behavior a bit frustrating - before, you would toggle between the current app and the most recent; now, you toggle across the dock, left to right. I'll have to give Keyboard Maestro a tryout!]

