I recognize this discussion has been dead for months, but I've been looking for a long time for fix to the same issue: I have 8gb RAM and no matter what I do, no matter what apps I run, within an hour all my memory is taken up. I don't actually see a huge performance hit, but it does annoy me to see more than half my RAM taken up with inactive ram.
Recently, however, I've found 2 great ways to clear the inactive RAM:
1-A free app called MemoryKeeper. It's in the App Store. You can run it, and it will clean up your RAM (pressumably the inactive RAM), and tell you exactly how much it cleaned up. Plus, doubly exciting is you can choose to have it do so automatically for you, regularly throughout the day. You can tune the automatic cleanup in the preferences.
2-open terminal, type "purge" and it'll take maybe 20 seconds to clean your RAM. I read that it'll clear every last bit of inactive RAM, which can temporarilly hamper performance for programs currently running, but I don't really notice any performance difference. And yes, I'll reiterate that all you do is open terminal and type the one-word command. As simple as that. I have heard that this function doesn't work on older OS X versions unless you download the "developer tools." It runs on my machine perfectly (Mountain Lion) and I have not downloaded the "developer tools."
This article is a must-read:
http://howto.cnet.com/8301-11310_39-57602426-285/speed-up-your-mac-with-free-app -memorykeeper/
This is good too:
http://www.electrictoolbox.com/purge-free-inactive-memory-mac-osx/
Actually this is probably unneeded but I'd rather give too much than too little:
http://www.electrictoolbox.com/free-inactive-memory-mac-freememory/