I'm huge on keyboard shortcuts. I mean: huge. I dig through Quicksilver's documents whenever I have time to learn more about all the insanely handy things it can do, and I'm still waiting for a "Keyboard Shortcuts and You" book to show up at Amazon.
One thing that's always irked me is, even though OS X and most of the apps I use are fairly well thought-out in their design and use of common shortcuts, there's one option I use on a frequent basis in a lot of apps that's never had a shortcut: zoom, also known as the green "best fit" button in the top left of nearly every window.
Earlier this week I noticed this option is located in *nearly* every app's menu under Window > Zoom and I realized: why don't I just assign my own shortcut? While this tip might sound like a no-brainer to some, I've met enough people who had the "oh that's so obvious! Why didn't I think of it?" reaction, so I figured I'd post it: if you want a system-wide shortcut for this often-used feature, (or a shortcut for almost any other app's function that doesn't already have one) just head into the Keyboard and Mouse System Preference pane in either Panther or Tiger and open the "Keyboard Shortcuts" panel. Click the plus sign to add your own shortcut and make sure "All Applications" is selected in the dialog, then type in "Zoom" for the menu title. Chose your own shortcut wisely so it doesn't conflict with other applications and *bam* - you now have a handy keyboard shortcut for an often used but sorely overlooked feature of OS X that should work in almost every app, even Carbon ones like Firefox and the Finder. You'll probably need to restart any running apps for it to take effect, but if you did this correctly your new shortcut should show up under the Window menu as in the picture accompanying this post.