Huge memory leak in Mountain Lion
Any memory usage that you can get rid of with purge is, by definition, not a leak.

OS X has always tended to use memory for its disk cache, and newer version have become fairly good at keeping memory busy with caching when not used actively by programs. When a program uses that memory for computation, the amount used for the disk cache is used correspondingly.

Why do you think it’s bad that your memory is used? Unless you’re seeing performance degradation, it tends to be a feature to effectively use your installed RAM.

ETA: What @zigg said above.