One of the things I'm sure we all love about Mac OS X is how integrated so many of the apps and services are with each other, but did you know that integration can stem all the way down to the fonts and colors you use amongst your apps? In almost any input-based, Cocoa-written app you're running (Firefox, for example, is not written in Cocoa), you can press cmd + t to open a simple, unassuming fonts palette that you've probably seen at one time or another. But if you chose a particular font and size that you like in one program, you can click on the gear in the bottom left of that panel and chose 'Add to Favorites' which places it in a category aptly named Favorites on the left side of that panel. The beauty of this is that any other program that has access to that system-wide fonts palette can also make use of the fonts you mark as favorites. For bonus points, click and drag the dot at the top of that panel to reveal a preview area (pictured) where you can see what your font is going to look like before running with it.
Next up is the Color palette, accessible with the cmd + shift + c shortcut. This palette employs the same basic concept: you can use it to find a color you like, and then drag a swatch of that color to the white squares at the bottom of the panel to save a version and share it amongst your other Cocoa-based apps.
These little built-in tools can be really handy when working across many apps in Mac OS X. You can set a favorite font in Yojimbo (or your choice of other junk drawer apps), and then use it when chatting with a friend in Adium or iChat. Use a favorite color for highlighting in OmniOutliner? Why not save it for the next Mail message you have to send, or those Final Cut Pro and Motion projects coming down the pipeline?
By no means are these a revolution feature of Mac OS X that'll rake in the switchers, but they might just make your daily activities go a little bit smoother.