Of course, there are many apps that do this, including Instapaper and Evernote. Instapaper gets the job done via a bookmarklet; just give it a click while at the desired URL and it's saved. Evernote does pretty much the same thing, thought their bookmarklet behaves differently between Safari and Firefox. In addition, they have the Mac application for tagging and organizing.
I spent the day using Quiet Read instead of the other two. Here's what I liked and disliked. First, adding a URL is as easy as possible. Simply drag it out of your browser's address bar and drop it onto the menu bar icon. The display keeps track of how many you've collected. A tidy drop-down lets your browse the saved articles for easy selection.
What I disliked is that once an app is gone, it's gone. Instapaper saves read articles.
But Quiet Read isn't Instapaper. If you're looking for a free, simple way to save articles for later reading (and you're running 10.6 or later), Quiet Read could be the solution.