The Silver Searcher – Faster than Ack. Better than grep.

FSEvents tools – Some command-line tools for working with OS X's filesystem events. Similar to inotify-tools.

LS_COLORS generator – For Linux, OS X, and *BSD.

Twisted hang detector – Find out what's causing the reactor thread to hang.

Memex Trails – A Chrome extension to help you figure out the path you took to get to a site.