Unlike perhaps any game since the 16-bit era, Team Fortress 2 has a look that is almost timeless. To get characters looking that good takes not only years of development, but years of concept art as well.
The pieces in this gallery are all taken from those years of art and development, some of them clearly showing the way towards the game's iconic final designs, while others...don't.
What's really interesting seeing in the development of some of these characters is that the core concept was there early on in the process. The Demo Man, for example, may have changed outfits and even races more than once (at least when the "current" art style and direction was settled on), but he's always been Scottish and he's always been one-eyed.
You'll see that continuity in a lot of the characters and locations in this gallery. And if you'd like to read about the game's art design, here's a great feature with some of the guys involved.