Cathode, a vintage CRT teminal display application for the Mac, has one of the most ingenious and well executed licensing schemes I've ever seen.
The app is free to download and use, but the display will degrade over time if you don't pay for the application. Soon the screen is filled with static, the horizontal hold starts going haywire and everything becomes pretty hard to make out.
Where most trials simply time out, they've built a system for a paid upgrade that is actually feels like a natural limitation of the technology they are graphically mimicking. It makes for a beautifully integrated incentive for upgrading.