Synapse Is a Super-Fast, Tightly Integrated Application Launcher for Linux [Downloads]
GNOME-Do is still one of our favorite Linux launchers, but it hasn't updated in over a year. If you're looking for something a bit fresher (and without the ugly Mono dependencies), Synapse is a great replacement. It's similar to GNOME-Do and other application launchers in the sense that, with a quick keyboard shortcut, you can launch an application or take action on a certain file, depending on the plugins you have installed.
However, while the plugin list isn't quite as extensive as the more mature GNOME-Do, the Zeitgeist plugin allows for a lot of cool things. Zeitgeist (which comes pre-installed on Ubuntu) is a service that logs all your activity—files opened, websites visited, conversations held—and all these are quickly available through Synapse. You can look up recently used files (say, if you closed that document by accident or want to repeat the song you just heard) and even find other similar files. It's a slightly different approach to quick launching, and one that may have a slightly bigger learning curve but it has a ton of possibilities since Zeitgeist logs so much.
Apart from all that, you also have the usual plugins—Banshee, Rhythmbox, Dictionary, Log out/Shut Down/Suspend, and quick Terminal commands, to name a few. If you rely heavily on some of GNOME-Do's more custom plugins, you might have to wait a bit before Synapse fits into your workflow better, but if not, it's definitely worth checking out. Hit the link below to read more.