After a comment in a recent entry, I had my first experience playing with Butler today, and I think I am starting to see why so many people rave about it. This donationware software is now up to version 4.0b15 (it was formerly known as Another Launcher). Why did I wait so long before trying it? Mostly because I didn't like the sound of the name (either one) ;-)
Butler Basics
The basic idea, that of a keyboard-based application launcher, is similar to LaunchBar and QuickSilver, however there are some extras thrown in. Launching an application is a matter of hitting the hotkey (ctrl-space by default) to bring up the launcher window and typing the first few letters of your desired app. Folders, web bookmarks and email addresses are also recognised and will open the appropriate item (or compose a new email, as appropriate). The search algorithm seems much the same as LaunchBar's. Both are better than QuickSilver. The launcher window snaps into existence when you press the hotkey and looks pretty good too. Speedwise Butler is definitely faster than QuickSilver on my Mac. It may even be faster than Launchbar.
A myriad of extras
If Butler has a drawback, it's that it does so much. The configuration and preferences windows are quite intimidating at first. In addition some of the extras seem a bit superfluous. Surely the whole point of a keyboard launcher is to use the mouse less, so the inclusion of a launcher menubar item, a bookmarks menubar item, and a web search box in the menubar comes across as a little odd. They look pretty, but in my opinion don't add much convenience and take up more space in my already crowded menubar (although you can choose to put all the menubar items into a docklet if you prefer). I can only assume that some people (ones who can't type, presumably) use Butler for these menubar items and don't use the keyboard functions at all.
Another Butler extra is controlling iTunes with hotkeys (play, previous, next, etc.), and there is also the option of enabling a bezel which fades in and displays the track details each time a new song starts. This level of iTunes control would usually be the result of a standalone app or control panel (PTHiTunesNotifier (RIP) was an early example, and there are dozens of Konfabulator widgets to control iTunes in the same fashion). However, unlike both QuickSilver and LaunchBar, Butler does not recognise iTunes playlists (not an issue for me, I don't use them).
In my opinion by far the best extra is easily Butler's web search hotkey. This very nice feature is faithful to the whole keyboard-launcher concept. Press the hotkey (Ctrl-Opt-W by default) and a white bezel appears centered on screen containing a web search box. Just type away and hit return, and watch your Google or Dictionary.com results appear in a Safari page. I can see myself getting a lot of use out of this one