A way to link post titles (not pages) to external sites?

I’m looking for a way to easily link post headlines to external sites, much like how Tumblr and Daring Fireball’s linked list work. I don’t mind much how it’s implemented, so long as it’s fairly simple and accessible from the Press This bookmarklet.

I don’t think this feature is included in WordPress by default, and I’m having trouble finding a plugin that does it. Anyone have ideas? Thanks a bunch in advance.