AlternativeTo is I site (beta) I first read about on Lifehacker. AT is a site where you can find any alternative application to what you are looking for. For example, if you use Windows but need something similar to the Mac app, Garageband this is where you would go. You need an alternative to Photoshop, Picasa, Jing, Comic Life, this is the place!!! Also, you can search via different operating systems such as: Windows, OS X, Linux, and even online resources.
I view AlternativeTo as Version Tracker on steroids. This really is a very useful site and has a very intuitive user interface. I highly recommend checking this site out and see how easy/beneficial this site is to use.
Below is a pic of a search I did for apps similar to iTunes.
For more info click here.