Using what are called Services, you can do all sorts of crazy, incredibly handy stuff. One of my personal favorites is creating a keyboard shortcut so that every time I highlight some text, I can press that shortcut to automagically start a new email and include the selected text. It's pretty simple to set up, too. To get started, open System Preferences > Keyboard, click on the Keyboard Shortcuts tab, and select Services from the menu on the left.
Then scroll down in the rightmost pane until you see New Email With Selection (it's under the "Messaging" header), and make sure it's toggled on.
Note from my screenshot above that when you click on an item from the list, you can assign a keyboard shortcut to it just by clicking "Add Shortcut" and then pressing the keys you want to use.
I've used Control-Option-Command-M here, but you can do whatever you want. Just make sure you don't override an existing shortcut that's useful to you.
Now all you have to do is highlight any text and press your assigned shortcut, and you'll get a new message in Mail with the selected text included.
You can use this in Safari as I've done above, in TextEdit and Pages (and Microsoft Word!), and even in some pretty bizarre places, like any fields in the Contacts program. In fact, it'll work almost everywhere. If you do have trouble getting it to function immediately after you set it up, though, quit the program in question and reopen it.
One more neat trick here—when you import text into an email message in this way, it pulls in all of the formatting, including links, images, and fonts. If you want to strip out all of that in one fell swoop, just press Shift-Command-T (Format > Make Plain Text) when you're looking at the message in Mail. That'll change stuff I copied out of an article from this…
I use this pretty much all of the time. There are other ways of getting text into a new email message (such as copying and pasting, obviously), but I think this is the quickest method. Plus, I got to create my own keyboard shortcut, which is just darned awesome on its own!