A couple of neat tricks that I use from time to time:
$ mdfind: How could I be a Mac user for almost 3 years without knowing that you could use spotlight from the command line!?
$ purge: Flushes the disk cache. Usually, you don't mess with that stuff and just let do the OS to do its magic. Unfortunately, since installing Lion I sometimes see a huge performance degradation when many applications are being used at the same time. By running this command, after a brief pause everything returns to normal.
$ dnscacheutil -flushcache: I believe this is self-explanatory. Useful when useless information is obtained from a flaky proxy server and cached.
$ networksetup -setdnsservers Wi-Fi 18.104.22.168 22.214.171.124: Use Google's Public DNS servers (see situation above)
$ networksetup -setdnsservers Wi-Fi empty: Go back to default DNS provided by DHCP.