Mac OS X: Chuck is a very simple, extremely lightweight, and - as a result - super-fast application launcher. It's not going to please hardcore users, but will win over people who just want to launch apps from the keyboard as fast as possible.Why would you use Chuck instead of Alfred or Quicksilver? Because both apps have their moments where they lag and both apps provide a lot of features people don't necessarily need. If you do need those features, both are good choices. If your priority is speed and low impact on your system, Chuck may be what you're looking for.
If you've used an application launcher of some kind before, you know how to use Chuck. You hit a shortcut to bring it up (there is no default - you have to set it yourself the first time it runs) and just type what you want. Chuck's results are instant, it never lags, and if waiting half a second for it to fade in is too long for you then you can just have it appear instantly. If that's not enough, you can create shortcuts to open apps really fast. For example, typing "p" could open Photoshop without the need to type its name entirely.
The bottom line: Chuck does everything it can to save you precious fractions of seconds when launching apps. It's a free download on the Mac App Store. Plus its icon is a moustache, which is hard not to like.