Looking at your Free RAM is a useless measure, as the goal of any operating system should be to take full advantage of all the RAM it can. Otherwise it is wasting your money because you paid for something the OS is not using, and worse, it is not taking full advantage of the best resource for allowing your system to run at its faster.
The operating system should be giving RAM to any process that needs it, and if the applications do not need it, the operating system should be using that RAM for a file system cache so you can avoid very expensive disk I/O. And by expensive, I mean expensive in time, expensive in power consumption, and expensive in wear and tear on your disk drive.
On Mac OS X, you want to look at both Free and Inactive memory to see how much memory is availble for processes to use. Inactive memory is often used a a file system cache. But Inactive memory may also contain the code for a recently quit program so that if you start that program again, it can quickly be re-activated from the cached code already in memory.
As Linc has suggested, you want to look at Activity Monitor and see if there is an app or daemon that is consuming lots of Real Memory.
If you think your Mac is using too much memory, then you might also run an Applications -> Utilities -> Terminal -> "sar -g 60 100" command, which should let you see your pageout activity once a minute for 100 minutes (adjust the numbers to suite your tastes). If mostly you have zero or very low numbers, you are NOT suffering from lack of RAM. If you have a few spikes, that might come from starting a new app, or switching between apps. If you have sustained pageout activity, that might indicate the work you are doing it stressing your available RAM and might benefit from running fewer concurrent apps, or getting more RAM for what you are doing.
It is very rare for the operating system itself to be the cause of excessive RAM usage. More frequently it is an app or daemon that gets memory hungry.
Also Linc's suggestion to boot in Safe Mode is an excellent idea, as that will eliminate any 3rd party drives, kernel extensions, or daemons. If the problem continues when running in Safe Mode, then that would increase your argument that it is something Apple provided. If the problem goes away when booted in Safe Mode, it tends to indicate there is a 3rd party driver, kernel extension, or daemom that is causing the problem.
