I like elementary' s Slingshot, but as you can see from this screenshot, it doesn't correctly show all of my applications (you can see Synaptic on the Unity Launcher but not in Slingshot; generally all the apps in the Administration and Preferences menus are not featured). I tried lxlauncher in the past but I would like something that can be summoned with a hotkey. I know KDE has options under the form of widgets, but I'm a Gnome guy. Thanks for your suggestions!
gnome-do is a great tool, but like you said not quite what you're looking for? The only other things like you describe that I have seen are applets/plugins for docks such as Avant Window Navigator and Cairo Dock.