Fortress Forever is a mod which brings gameplay from Team Fortress Classic onto the source engine. While it has been out for several years, I only today read enough about it to install it and give it a shot.
Here's a video showing some of the game in action:
After installing, I found 23 servers online but none with any players. Seeing as how it is 4 AM, I am hoping there might be players online during other times of the day. We'll see.
In the meantime, I created a side-by-side comparison of the classes in FF vs how they appear in TF2. Sorry about the quick MS paint job.
Click for image --> FF vs TF2 character comparison