The folks over at Many Tricks have put Witch on sale for US$10 (normally $19) until 7 AM Pacific Time on Friday, January 28.
Witch shows you not just which applications are running, but it also shows you all of the different windows that are running in those applications. For example, right now I have three different windows open in BBEdit; if I use Cmd+Tab switching to move between applications, I can switch to Safari and then back to BBEdit, but Witch doesn't just let me switch to BBEdit, it allows me to specify that "I want to switch from Safari to the BBEdit window that has the article I'm writing about Witch."
If you haven't seen it in action, you can download a free trial from the Many Tricks website, which also has some screenshots available.
Witch is available for purchase either through the website or through the Mac App Store.