A lot like Superuser but more user-friendly if you ask me. It's supposedly 'better' than Superuser, but the only difference I've noticed is that the popup dialog is less of a hassle than SU. I like it.
Manage ROMs (upgrade, flash, etc.) without booting into recovery each time. Free version is pretty much useless by itself - this one's definitely the way to go if you want easy access to ROMs and mods.
Huge selection of custom fonts to choose from and the app itself is actually pretty lightweight. It might take a few minutes for the all the previews to load right after installation, but after that it should run pretty smooth.
If you're rooted, odds are you're overclocked and have a ton of other performance tweaks. See how you stack up against other similar devices. (No root required, but it's pretty fun to use as a root user.)
Every time you flash a new ROM you lose all your WiFi networks, which usually means asking everyone for their password every time you go back to their house. With WiFi Key Recovery, you'll never have that problem again.