Without the need for third party applications, you can bind launching of any application to a keyboard shortcut using Services.
Open Automator.app, create a new Service.
Set it to receive no input, and from the Library, drag "Launch Application" to the editing panel. Select which application you want to launch.
For example, I used Preview.app.
Save it under any name you want. Now, go to System Preferences » Keyboard » Keyboard Shortcuts » Services. Find your new Service under General and enter the keyboard shortcut you want to launch the application with.
For example, ⌘-1.
Be careful though: Most applications override keyboard shortcuts with ⌘ and numbers, since they have special meanings. For example, in applications using tabs, this will select the first one. In Eclipse, this opens the quick fix menu, et cetera.
To change Services you already created, go to Library/Services.