My personal favourite is the Purge Memory button.
It may not really do anything, but is sure makes me feel good when something appears to be happening.
I always press this little fella before I do a final render. (and cross my fingers, pray to Allah and say 6 Hail Mary's that I won't get the "out of memory" message.)
BTW, what does "Purge Memory" actually do? Purge your memory I suppose.