To elaborate, in case you don't feel like reading through the Qt site, Qt is a software toolkit originally designed for building GUIs (Graphical User Interfaces - basically, what gives you pretty pictures to poke at instead of typing text commands at a prompt :P), although it now does a lot more. One of its main advantages is that it works on all major desktop OSes, some mobile OSes (and more on the way), and a whole handful of others, some of which are rather obscure. The upshot of this is that most programs written with Qt are easily portable to any platform Qt supports, so you don't have to deal with problems of program Foo being available for Windows, but not for Linux/Mac (or vice versa).
Nokia bought Qt from a company called Trolltech a while back, and has been busy making it work on lots of new platforms since then, including Maemo. That, (among other reasons) is why it's the New Hotness around here.
Some examples of apps written with Qt:
Open Source: Lots of Maemo apps, the KDE Software Compilation & associated apps (kde.org), lots more that I forget because it's 3am (check qt-apps.org for some others)
Proprietary: Google Earth, Skype (some platforms), Opera
Heck, I've seen pictures of it running on a coffee maker!
I hope that explains things for you.