Too Much Inactive RAM, Too Little Free RAM

I used to have a 2007 iMac running Snow Leopard, and I switched to a 2011 iMac running Mountain Lion. On both of my computers, I kept running out of free RAM after a while, and my inactive RAM kept taking up all my free RAM. It still does it. I end up with around 30MB of free RAM, a reasonable amount of active RAM, and a massive amount of inactive RAM after a few days, then my computer slows down a lot.

I have to keep flushing the inactive RAM with a memory cleaning tool or else my computer slows to a crawl. Repairing the permissions is supposed to clean out the inactive RAM (thanks, Tuttle), but alarmingly, this doesn't work for me. There must be something causing this problem, and I want to find it. How do I do this? Can processes currently running be using inactive RAM, or is that only possible if they have just been closed? Is there a tool that can check how much inactive RAM is allocated to each process?

P.S. To make it clear: I know that I'm not supposed to worry about inactive RAM and let the system do its own optimization, but it must not be doing it right because it's causing extremely noticeable slowdowns. The computer is like a Geo Metro pulling a whale if I don't flush the inactive RAM periodically.