One of my favourite utilities is Launchbar, which gives me instant access to pretty well anything on my Mac by simply typing a few letters. Michael Tsai’s review: ATPM 11.03 – Review: LaunchBar 4.0.1 provides a good explanation of what Launchbar does and how it works. His conclusion is that Launchbar’s the best.
There are other utilities that can do a lot of what LaunchBar does, but I think LaunchBar is the best. It’s not that LaunchBar has a killer feature that the others lack, but that to me it seems more refined. The main widget is compact and unobtrusive. It searches faster. The abbreviation algorithm is smarter. The preferences and options are presented more clearly. Features seem to fit together naturally, packing a lot of power into a simple interface.