The Spotlight feature in OS X can start any application from the keyboard, and you don’t even have to program it. But if you want to assign a specific keyboard shortcut to an application, that capability is not built into OS X so you’ll need to use additional software for that (see the comments at the bottom for a discussion).
And if you came here looking for an Activity Monitor keyboard shortcut (as my blog stats suggest), read all the way to the bottom to learn why you might not even need one.
(Update: If you really want to assign a keyboard shortcut to a file like you can in the Properties dialog in Windows, conroy in the Adobe User Forums suggests an OS X tip involving Automator and Services.)
The secret weapon is Spotlight
With Mac OS X 10.4 or later, you don’t need to create or even learn shortcuts for applications. They’re already there, but not in the form you may be expecting.
The key is Spotlight, the search utility built into OS X, which is a decent application launcher. Just hit the Spotlight keyboard shortcut (Command+spacebar unless you changed it), type the first few letters of the application’s name, and if the application’s name is the Top Hit, press Return to launch it. (In OS X 10.4, you need to press Command+Return; In 10.5, Apple simplified it to just Return.) If it isn’t the Top Hit, use the usual Spotlight shortcuts to get to it in the list: Use the up and down arrow keys either alone, or with the Command key to jump categories.
So for example, if I want to use Activity Monitor, I press Command+spacebar, then type “act” and boom, there it is. Depending on which files and applications are on your computer, Activity Monitor may appear before you get to the third letter.
If you’re annoyed because you have to type a few letters before Spotlight narrows it down to the application you want to launch, have patience. If you keep picking the same item from the search results, OS X will eventually turn it into the Top Hit, and over time you’ll need to type fewer and fewer characters to get your preferred result. (You can accelerate searching for multiple words through abbreviation; read about it here.)
By now I’ve got my Mac trained so that after pressing the Spotlight shortcut, Photoshop becomes the Top Hit as soon as I type “p”, and Mail is selected as soon as I type just “m”. Yeah, it’s more than a single keystroke, but on the other hand, to get this feature I didn’t have to modify my system, spend an hour configuring shortcuts, or add a utility such as LaunchBar. Spotlight is already capable of launching any application on your system without any further setup.
There’s a second benefit to leaning on Spotlight for this purpose: You never have to dig down to open an application or utility that isn’t already in the Dock. You don’t even need to know where it is on your hard drive! Using Spotlight as an application launcher can also let you reduce the number of application alias icons littering your desktop or Dock. Since I usually launch via the keyboard, I actually have my Dock hidden by default.
I took it one step further and used the Keyboard Shortcuts customizer in the Keyboard system preference to change the Spotlight shortcut to F12 on my notebook and F13 on my desktop so that I could get to Spotlight by pressing just one key.
Before OS X 10.4 brought us Spotlight, I used to be a devotee of LaunchBar, and I tried Quicksilver. The problem is that even if a launcher app is free, the second indexing engine drags on the system and adds complexity, another database to store and manage, and removes another set of keyboard shortcuts from the pool. When Spotlight came out, I realized that it does most of what I need, and well enough. The things Spotlight can’t do that the other utilities can do I’ve mostly covered by having Spotlight trigger AppleScripts…but that’s a subject for another entry.
You don’t really need to launch Activity Monitor
Update: There are so many Web searches that come here looking for an Activity Monitor shortcut that I have to add this: If you frequently want to monitor Mac system status information, you should download iStat Menus (used to be free, I decided to pay) atMonitor, or MenuMeters (those last two are both free). They put all of that CPU, RAM, drive, network, etc. information right up there in your menu bar. Ultimately, iStat Menus is why I don’t need to open Activity Monitor, because what I want to see is already visible or accessible in one click, and without blocking what I’m working on.
In the picture above, you can see part of my iStat Menus setup in the menu bar: current network throughput, hard drive activity and free space, RAM usage, and multi-core CPU meter. Clicking any of those items drops a menu with much more detailed information.