I've created a few iOS and Android apps for mobile devices and tablets, but just a few days ago, my first 'desktop' app (for Mac OS X 10.7+) was released on the Mac App Store.
I created the app mostly because I realized I would often drag an image to Photoshop to simply resize it from something like 1024x768 to 500 pixels wide, constrained. Quick Resizer makes resizing JPEGs and PNG files super-easy, and everything can be done from the keyboard (or the mouse, if you're so inclined).
Developing for the Mac is a bit different than the iPhone, but Objective-C and the libraries included are very similar, so it wasn't like creating the first iOS app!
If you like the app, feel free to rate the app on the Mac App Store after you buy it :)