Quicksilver defines the way I use my Mac. I use it to launch applications and scripts, open files, switch network locations, send emails, control iTunes, and far more, all with no conscious effort, and in a fraction of a second.

Latest version: now found at qsapp.com (at github choose the "package" download) or at blacktree.com.

10.6 Stability problems: due to old plug-ins, I think—go to ~/Library/Application Support/Quicksilver/PlugIns and try removing some before launching QS. Can re-add them one by one, see what crashes QS. For me, it was AirPort Module.

Sluggish? I think this is entirely due to the number of items in the catalogue database. If you have upwards of 100,000 items, searching it will be slow on slower machines! This was certainly my problem on my old Mac. Reducing number of files being scanned (e.g. omit Library folders!) made QS snappy again.