Both Borderlands games have decent expansions, wasn't keen on the Moxxi one on the first game with it being arena based but other than that they have been great on the whole.

Fallout New Vegas like mentioned previously, my favourite has to be Old World Blues, just like an old sci fi movie and setting, absolutely awesome stuff.

Elder Scrolls Oblivion is worth going back to if you only have the vanilla version, Knights of the Nine and Shivering Isles are 2 excellent expansions. Shivering Isles, if you do all the side quests as well as the main one will give you at least an extra 20 hours playtime.

Deus Ex Human Revolution Missing Link gives you an extra few hours play and fills in a bit of a gap in the story that happens towards the end of the main game when you are heading to the games final destination.

Yeah, it has to be a problem with Steam version. It's pretty common that expansions and base game share same files on steam servers and have to be downloaded anyway.

The worst example is Civ4 complete on steam. Beyond the sword contains the base game and the first expansion, but NOT scenarios. If you want to play base game scenarios or from first expansion, you have to download them anyway. Two times more space for few extra maps.

Fallout: New Vegas: Old World Blues is probably the best in recent memory. I don't like Borderlands, but I hear those are crazy good if you do.

The expansions to Neverwinter Nights 2 are widely considered to be even better than the base game.

Warband + Napoleonic wars

I hated warbands multiplayer, I always thought it was just a game of luck rather than skill. Napoleonic wars almost eliminates melee combat (which was THE issue for me), brings a new setting to the game and more game modes.

Bloodmoon for Morrowind, you should play this one before the Skyrim expansion, Dragonborn, comes out.

A couple old strategy games, Empire Earth and Age of Empires 2 had really good expansions. If you haven't played them and you enjoy strategy you should take a look.

Gothic 2: NIght of the Raven is a decent expansion and adds a decent amount of gameplay, worth a look if you enjoyed the series.

Homeworld: Cataclysm. This is one of those expansions that you really loved or you really hated. Since the second game was received so badly it's safe to assume that many fans of the original really loved Cataclysm.

Silverfall : Earth Awakening. Also rework the base game mechanics for the better, it was almost unplayable without it, truth be told.

Has anyone played Gods and Kings? Is it worth getting?

The changes to the gameplay are not earthshattering, and at its launch price, it was a bit overpriced, but if it is on sale, then yes, go ahead and get it! It adds more options for the player, more viable strategies, and that is always a nice thing.

i loved the expansion for age of empires II
the aztec nation and campaign were my favorits of the entire game

Wages of Sin expansion for Sin 1 was good. So were both expansions for Quake 1.

Neverwinter Nights 2: Mask of the Betrayer is easily much better than the basegame.

It is worth getting you already like Civ 5. It offers new options that make the gameplay more engaging, especially at the early stages, in my opinion. But if you didn't like Civ 5 in the first place, or if you still wish it was more similar to Civ 4, then forget about it: the expansion adds new cool stuff, but it doesn't fix any of the old issues.

Definitely this one, especially for the contrast between it and the main game.

New Vegas also had excellent story DLC, I'm not sure if they're long enough to count as expansions though.