With Quicksilver,you can just type the name of the application you want to launch, you get a nice little interface to tell you if your on the right track (like T9 for applications)
Basically what we have here is a movement towards leveraging a text interface in a gui environment , which I support.. It's true you could run a dos window and launch things from there.. but there's autocomplete issues.. are you going to put every app you have in your path.. etc.. It's just not practical.
Enter Colibri , it doesn't currently have all the functionality of Quicksilver (neither will it likely). but it does have some very nice features.
I like to use the windows calculator, for example; now, I know the name of it 'calculator' . Why should i have to get my mouse, possibly go through 2 - 6 levels of menu (depending on how committed you are to it) , to click on the tiny little icon, not to mention the popping closed start menu flyouts that seem to fight you all the time. With Colibri I can just type the name 'calculator' , and in fact i can get it in just 'ca' , that's a lot easier.
Click on the screenshot below to see a screencast I made of Colibri in action. Colibri is launched via hotkey (ctrl-space in my case) , in this screencast, none of the commands (other than closing some apps) is done with the mouse, just ctrl-space, then start typeing, maybe arrow up/down and enter.
There are some shortcomings, for example I dont know how to add something that isn't in my start folder to its list.. maybe it's possible I don't know..