Mac OSX is beautiful, not only in its design, but also in its memory management. However, sometimes the inactive memory takes long time before released.
According to Apple support Inactive memory is
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.
In many cases you will need to jump from a heavy application into another, and then you will need either to accept the system performance, which is not deadly bad, or use a simple command line to free your memory.
First, how to monitor your memory?
Mac OS X comes with Activity Monitor Application that gives you a glimpse about your system resources, one major resource is your memory.
Read Apple help on Reading system memory usage in Activity Monitor
Now, how to free the inactive memory without having to reboot?
Open your terminal app and run
Checking the ManPage of Purge, purge task defined as
force disk cache to be purged (flushed and emptied) with a description
Purge can be used to approximate initial boot conditions with a cold disk buffer cache for performance analysis. It does not affect anonymous memory that has been allocated through malloc, vm_allocate, etc.
No need to download an app to free your memory and moreover, you do not need to buy an app for that purpose, it comes for free.