What is the OS X equivalent to the free command to show buffer and cache memory?
vm_stat may just show you what you need. As per the man page:


  vm_stat -- show Mach virtual memory statistics


It can show you the following: 


Pages free
Pages active
Pages inactive
Pages speculative
Pages throttled
Pages wired down
Pages purgeable
"Translation faults"
Pages copy-on-write
Pages zero filled
Pages reactivated
Pages purged
File-backed pages
Anonymous pages
Pages stored in compressor
Pages occupied by compressor
Decompressions
Compressions
Pageins
Pageouts
Swapins
Swapouts