My MacBook Pro uses an extraordinary amount of RAM
It's probably just been a while since you restarted your computer. When you close a program, the memory it was using doesn't go back to &quot;Free&quot;, it gets marked &quot;Inactive&quot;. It's still available for use by other programs, though:

Inactive memory
This information in memory is not actively being used, but was recently used.
For example, if you've been using Mail and then quit it, the RAM that Mail was using is marked as Inactive memory. This Inactive memory is available for use by another application, just like Free memory.  However, if you open Mail before its Inactive memory is used by a different application, Mail will open quicker because its Inactive memory is converted to Active memory, instead of loading Mail from the slower hard disk.