Panzer Elite and Freespace 2; not only are they great games but the amount of content available through the modding community make them both truly amazing value for money.

About time someone mentioned that...

I am actually quite sad that no one mentioned Die by the Sword but me.

I would love to, but controlling that game was like having sex in the middle of the winter on the highest hill of the country with no clothes and no copy ... Err, reproduction protection on, whilst being 2 km away from your partner and ... Well I think I've made my point clear :D

Legacy of Kain: Soul Reaver. Oh wait... It's not available here yet (go vote for it on the wishlist).
In that case for me it would be a tie between Gothic series and Heroes of Might and Magic 3.

Wasn't Operation: Flashpoint a GOG title for a while? If so, that one gets my vote though I didn't buy it here.

If you're a fan of all things shooting then either of the Unreal Tournaments, the modding scenes for each seperate game is truly crazy big with said mods often being seperate, quality games in their own right, I mean it has spawned Red Orchestra, Air Buccaneers and Alien Swarm, to name a few, but there are far, far more due to EPIC actively nurturing the scene themselves with user friendly tools (heck, even I managed to make some nice levels) and competitions with enormous prize money at stake.
Heroes of Might and Magic III is also a good call as are all of the Fallout games, the RPG's providing a brilliant single player distraction whilst the 'Tactics' version gives a devilishly mooreish multiplayer option that's both lasting and very, very fun! :)

Sanitarium is the best !

I love RPGs, Fallout 1 & 2 just haven't aged well and 3 feels too much like a reskinned Oblivion. They're not bad games, they just aren't as good as Tactics.