The output of free -m is:
total used free shared buffers cached
Mem: 595 482 112 0 63 324
-/+ buffers/cache: 93 501
swap: 0 0 0
Which value of used memory is correct, 482 or 93?
You have 112 MB of completely free memory, BUT the 501 mb you see is without 'cached' memory. This means that the OS has put some stuff in your memory to be quicker. It calls this "used" (therefore your 'free' number is only 112), but it is actually available for you if you need it.
This is a good thing, because unused memory is useless memory. The cached memory can be cleared if needed. The old "I need to clean up memory" stuff people used to do for windows 95 isn't needed here: it's all fine and happy :)
The number you are looking for is 501 free (in megabytes because of -m).
see for reference these pages:
http://www.linuxatemyram.com/
http://www.itworld.com/it-managementstrategy/280695/making-sense-memory-usage-linux
