connects as user to host on port with Screen Sharing, and, more generally, "open URL" works for any URL scheme registered with Launch Services, including, but not limited to, the obvious ones.
One downside worth mentioning is that you can't read the full description of each available updates, like you can when you click on an item in the GUI version.
say is pretty useful and fun. When I'm compiling or just telling the Terminal to do something tedious, I add a say command at the end and it will notify me when it's done.
For example, if I want to install mercurial and git with homebrew:
brew install git mercurial; say "The task is finished, Master."
As an aside, in Lion at least, you can download high-quality voices for dozens of languages in Speech Preferences (including the ones Apple uses for Siri in several countries).
One of my favorite is pbpaste/pbcopy.
What's yours?