I found an AppleScript on GitHub for centering the frontmost window. It didn’t do quite what I wanted, so here’s my slightly-modified version, which will center the window in your screen horizontally without changing the vertical position or size.

tell application "Finder"
	set screenSize to bounds of window of desktop
	set screenWidth to item 3 of screenSize
end tell

tell application "System Events"
	set myFrontMost to name of first item of ¬
		(processes whose frontmost is true)
end tell

try
	tell application myFrontMost
		set windowSize to bounds of window 1
		set windowXl to item 1 of windowSize
		set windowYt to item 2 of windowSize
		set windowXr to item 3 of windowSize
		set windowYb to item 4 of windowSize

		set windowWidth to windowXr - windowXl

		set bounds of window 1 to {¬
			(screenWidth - windowWidth) / 2.0, ¬
			windowYt, ¬
			(screenWidth + windowWidth) / 2.0, ¬
			windowYb}
	end tell
end try

Paste this into Script Editor or download the script file. Put the file in ~/Library/Scripts. Make sure you’ve got the Script menu turned on in the AppleScript Utility application. It will show up in there, and should work in most Mac applications.

I have found a couple of limitations. It doesn’t seem to work at all with Firefox, which doesn’t have very complete AppleScript support. It’s likely that there are other applications out there with missing or incomplete AppleScript support that this won’t work for either. The second problem is that there’s a bug in the Terminal application that moves the window upwards, even though the script is written in a way that’s supposed to preserve the vertical position. It would be possible to work around this, but I decided that it didn’t bother me enough to clutter up the code. If you want a workaround, post a comment and I’ll give it a shot.

The original script doesn’t specify a license, so I hope it’s OK to modify and share it. I figure that, since it’s published on GitHub, and the author releases a lot of open source code under very liberal licenses, that he’s probably not intending to keep very tight control over it. My changes are offered to the public domain.

I’m just as obsessive with making sure objects are lined up straight on my (real life) desk top. Sadly, AppleScript can’t help me there.

Update, 15-Nov-2010: This continues to be a fairly popular post, which a lot of people seem to come across through search engines. See the comments below for tips about ensuring that the window size doesn’t change, and for centering vertically as well. I’ve also noticed that this fails on multi-display setups, centering between the two screens instead of on the current screen. I’ll try to come up with a fix for that.