Qt is a programming framework and in my humble opinion the best one around. It supports the major desktop operating systems and even one or two mobile platforms. This means that without having to change a line of code you can create graphically pleasing and useful programs for multiple operating systems.
When Qt was bought by Nokia I was concerned that it would be closed off. Luckily that didn’t happen and Nokia actually improved Qt. Then when Nokia went over to the dark side and became a windows supporter 😛 I became downright scared that Qt support for other operating systems would languish.