Show free memory.

$ free -ht
             total       used       free     shared    buffers     cached
Mem:          3.9G       3.8G        32M        96K        97M       1.1G
-/+ buffers/cache:       2.6G       1.2G
Swap:         1.0G        61M       962M
Total:        4.9G       3.9G       995M