If you use Process Viewer, you'll see some of the info you're asking for. Also a TOP command in terminal shows which apps are using memory, as well as memory usage by the system as a whole.
Memory usage is not really cleared after use in the same way that OS 9 handled it (thank goodness!) OS X handles memory usage much differently, assigning memory, but adjusting that memory usage dynamically. Even if you view how much memory is 'assigned' as in use, it changes as you watch, including paging memory out to the disk when necessary.
I watched top run on a system with 256 MB of installed RAM running about 40 apps at the same time, really crappy performance, TOP showed about 3.5 GB of virtual memory in use! But all apps could be used (useful?? not really, but working). When you close out all those apps, not all the memory is freed up, some is kept in cache, for later opening those same apps, as a performance enhancement. That's OS X and Unix working together to do its job!