I second Defense Grid

All the time, as necessary. Here are a few:

• screen recognition bot to automate tedious repetitive tasks in 2D Java based MMORPG game. (based on wxWidgets / OpenCV)
• Mutual Fund data aggregation with graphs to aid in choosing Mutual Funds for investment.
• Chore tracker to track which roommates are doing chores, with custom features not available on any online chore tracker I could find.
• Financial tracker similar to the chore tracker to track and encourage equal share in expenses by roommates.
• A quick little web app to allow entry and search of recipes by ingredients they use.
• A brute force tool to determine the most optimized set of folders (or files) to burn to a DVD and waste as little space as possible.
• An app to quickly sort duplicate files into good/bad folders from a music folder which is constantly filled with new files most of which are the same name as ones that were previously deleted. Names are stored in a database and user is queried to manually tag new names as good or bad for the next run on a new set of files.

And if you want to go really far back, how about autoexec.bat and config.bat files with built in menus to allow boot time selection of unique memory configurations for various DOS based games so they would actually run.

At one of my previous jobs one of my co-workers brought in his spiffy Apple Laptop. Coincidentally, we had just received wireless mice with the tiny usb transmitters. The rear of his machine was facing me and easily accessible, so when we was away from his machine for a moment I plugged it into one of his USB ports. When he returned he was none the wiser. Then when he got back to work I inconspicuously would hang out behind is his chair and randomly whenever he moved his cursor over a button, I would swipe the wireless mouse I was holding a little causing his cursor to drift a bit just off of the button causing him to click in a blank space. This had him going for several minutes since I didn't do it every time. It was a great hoot when he finally got wise that something was up and turned around to see me grinning widely. :D