[SueG]

Sorry to let this get stale before replying. Grant, you do it the same way I do, the habit was formed before clippings became available. But with a blank TextEdit window peeking out below the browser window, it is actually easier to drag and drop the highlighted bit onto the TextEdit window. The browser stays in the foreground and you can drag more stuff to the same window before saving it or using it in a reply or sending it on. When saved from TextEdit the file doesn't suffer the limitations of a clipping file.

[Nobody]

There are already so many solutions out there, but no one has mentioned the use of Stickies. With your text selected press ShiftCommandY. OS X will open a new sticky note containing the text you selected. The beauty of stickies is that you can quit the app but still have your data when you re-open it. Stickies also has an "Export Text" function if you want to export a quick file or note to a Windows user. Stickies accept RTF formatting so I assume they export in RTF, too.
Stickies get even more fun with Panther and Expose if you like to stack them on top of one another.