I think there are interesting examples of 'impressive' code in the world of games. The place to look is games for fixed hardware such as games consoles and older home computing platforms. The software to look for is the titles that came out towards the end of those platforms' lives.
For example, Elite on the BBC Micro crammed a 3D space combat game, a market simulation and a map of a whole universe into 32KB. Later the same game was crammed onto a NES. This feat involves hand-optimised assembly language coding in which spending a day to shave off a byte was considered worthwhile.
You can find similarly impressive works on all the 8 and 16 bit platforms. Also look at the sound and graphic demos from the Amiga scene.
The driver for all of this is that you couldn't simply solve the problem with more hardware. Consumers couldn't upgrade the hardware, and were demanding ever more sophisticated games.
In addition, these games are impressive because they were written for devices that weren't initially designed for games. Something as simple as 'smooth 8 way scrolling' was considered a major feature for a game, because the hardware didn't support it directly and programmers had to be really clever to achieve it.
It's possible that those days are over. Maybe someone will push the PS3 or Xbox360 to do something mind boggling, but it seems that you can sell games without pushing this hardware all that hard, so the commercial pressure to do so isn't that high. All you get is small improvements in graphic quality, or larger environments, or more detailed environments.
Increasingly even handheld devices are too powerful to prompt that kind of impressive low level coding. You don't need to twiddle bits to write a 3D game on a modern phone.
So, look for other places where the hardware is limited. The Mars Rover is a good example.