Apptivate lets you bind keyboard shortcuts to keystrokes. Not "keystroke" -- keystrokes, or as Apptivate calls them "hotkey sequences." The biggest drawback to assigning keystrokes to these kinds of shortcuts are the inevitable conflicts which will arise when dealing with system-wide keyboard shortcuts. For example, you want ⌘ + F to show the Finder, but every other app wants to use that shortcut to the "Find" panel. Apptivate uses sequences, such as Option/Alt + L (pressed together) followed by F to "launch" the "Finder" (I like to use Option/Alt instead of ⌘ for shortcuts as it works well on USA keyboards when writing in English. You may prefer something else).
You can, of course, bind individual keys to open particular apps. When I press F1, for example, nvALT launches so that I can jot down quick notes to myself. Likewise, when I press Option/Alt + B I get BBEdit. I dragged some URLs from Safari to the Finder and created some "webloc" (web location), which are essentially bookmarks that open certain pages. Then I created some custom "sequences" to open them.
Since I use Safari, I decided to make my Safari shortcuts start with "Option/Alt + S" followed by a single letter to represent the page. For example, Option + S followed by W brings up the Wunderground weather page for my city, Option + S followed by another S brings up the [s]chool closing page so that I can check for snow delays or cancellations.
I created some VNC location files (vncloc), and now I can use Option + V (mnemonic for "VNC") plus "I" to screen share to my iMac.
I created an Automator action to move selected files in Finder to the Applications folder, so now I can press Option + F ("Finder") plus "A" for Applications. I haven't used many Automator actions because I always found them to be a hassle to launch. This is so much easier.
What you see here are Apptivate's very minimal preferences. Both are disabled by default, but you should enable both of them, because they are wonderful features.
"Hide application if it is active" means that if you press the hotkey sequence for an application when it is already shown, it will be hidden. (Yes, you could use ⌘ + H but this helps reinforce the shortcut).
"Enable application quick peek" will show you the application only as long as you press and hold its shortcut key. Note this will not work with sequences. For this feature, the application needs to be bound to a shortcut which is either a single F-key or a key plus Command, Shift, Control, or Option such as "Option + Y".
Just want to peek at Twitter, but not get sucked into it? Press and hold the shortcut for your favorite Twitter app. When you release it, Twitter goes away. Want to look in your Inbox to see if that one email message is there yet? Press and hold your mail shortcut.
I've already used this to pop open Activity Monitor's main window to see what process was chewing up my RAM.
Why use more than one launcher?
As I said above, I have used LaunchBar for years, and I plan to continue doing so. All of the launchers that I listed do more than what Apptivate does. They are "related but different." For example, when I open LaunchBar and type "fi" it's going to offer me Finder, Firefox, Fission, File Juicer, and many others. Whichever one I have used most often and most recently should be at the top, but I'm never 100% sure what the first one will be. Typing "i" is even worse: will it stat with iTunes? iCal? iChat? iTerm? iPhoto?
With Apptivate, I can set a shortcut which will always launch the same application: Option + B will always show BBEdit; F1 will always launch nvALT, etc. On the other hand, I'm not going to set a shortcut for Fission, which I use only occasionally. For that I'm going to use LaunchBar. Apptivate is the launcher to use for your most often used files, Applications, websites, etc. Anything which can be represented in a file can be opened in Apptivate.
If you want to use the F-keys as shortcuts keys without modifiers, open System Preferences » Keyboard and check the box next to "Use all F1, F2, etc. keys as standard function keys."
Apptivate is a minimal tool which does its job very well. You'd be hard pressed to find another utility for that price which you'll use as often as Apptivate.
My thanks to MacAppsThatRock for originally drawing my attention to Apptivate.