LaunchBar is a smart launcher, which learns from your habits and patterns to help you work more efficiently.
Until midnight tomorrow (Sunday, eastern time) MacUpdate Promo has LaunchBar on sale for $17.50, which is 50% off the usual price.
I love LaunchBar because it's smart. For example, I use Mailplane instead of Mail.app, and LaunchBar has learned this, so when I trigger LaunchBar and type the letter 'm' it shows me Mailplane. If I type 't' it shows me Twitterrific but if I type 'te' it shows me iTerm.
It doesn't just let you launch apps either, you can also launch Preference Panes, create new events in iCal, search your address book (one of my favorite 'tricks': search for someone in my addressbook, navigate to their their phone number, and then press Enter. LaunchBar will show the phone number "full screen" like so:
You can also copy Address Book information from LaunchBar, use it to begin email messages, and more.
Using the "Instant Send" feature, I can select a file in Finder, then trigger LaunchBar and press "Command" twice (you can choose from several key commands to trigger Instant Send), and LaunchBar will offer to open the file in whatever application I search for next, similar to if you had right clicked on the file, chosen 'Open With' but your hands never touched the mouse. If you 'open with' your mail client, it will create a new message with the file attached. Or maybe you have an image that normally opens with Preview but you want to edit it in Pixelmator; Instant Send makes it easier and quicker.
LaunchBar also has a built-in calculator and clipboard history. Do you use search engines? LaunchBar will let you create searches right from within itself. It comes with a slew of search engines built in, and adding your own is simple.
Quite simply, LaunchBar is one of my absolutely essential applications. I've tried the others, and I always come back to LaunchBar.