Anyone know if any mods exist for this excellent game?

Alas, I believe there are none. There was a rumoured multiplayer add-on pack in the making, but that got cancelled as well.

I've heard about an add-on that was called Operation Stone Hedge, but I don't know any more about it than that.

If you browse through the history of the Terra Nova forum at TTLG, you'll find various interesting bits and pieces that have been posted over the years (most of which is pretty old by this point, mind, so I can't promise that things are still readily available). There are some higher-res bitmaps, level viewers, and even a level editor was produced, along with some custom missions. It's generally pretty hands-on stuff that you'll need some technical know-how to use, but a few talented people certainly have produced mods and utilities for the game.