I just learned about Mac Stickies, and they're pretty cool. If you're in a Cocoa application (like Safari, TextMate, and others) you can select text and/or graphics, and then easily save the content to a sticky note on your Mac desktop. To save the content you can either (a) remember the [Command][Shift][Y] keystroke, or else (b) click the application menu item (i.e., the "Safari" menu item if you're using Safari), then Services, then Make New Sticky Note.
The picture below shows what a Mac sticky note looks like when it's created on the desktop.
The content you selected is saved to a "note" that is kinda-sorta pinned to the desktop. You can move it around and do a few other things with it, but generally speaking, it's there as a sticky note so you won't forget whatever it is that you wanted. This is very cool, if not very well advertised.
Dragging images to the desktop
Okay, Stickies are cool, but some times you don't want a sticky note, you just want to save a picture or some text for some other reason. Again -- in Cocoa applications -- saving content to the desktop is super easy. If it's an image, just click the image, and then drag it to the desktop. That's all you have to do. The file is automatically created with the proper extension.
Dragging text to the desktop
If it's text you want, just select the text you want to save, then click-and-drag it to the desktop. The file is saved as a "text clipping" file on your desktop, and the name of the file is derived from the first part of the text you selected. Again, super easy. I just tested this in Safari and TextMate and it works just fine.