It's basically a complete overhaul of the game. It has its own (beautiful) world, some advanced scripting, comparing to Oblivion, and so on. For example, the starting area is just incredible, I don't get why Bethesda wasn't doing enviroments in this way.
Also, it seems to a bit less open-ended (it reminded me of Gothic in the beginning). Some of the beginner quests were also a bit strange.
But in general, I have to say it's an incredible achievement. If you didn't know it's a mod, you might not even recognize it :) I didn]t get too far, but it's definitely on my list of games to play.

Oh, and it also makes its own installation of the game, so it doesn't touch your saves, or mods for Oblivion itself :)

Oh man, this is the stuff I'm talking about. Any more information on these?

Pretty sure there was an unofficial campaign disc for IL2 Sturmovik, never saw it in the shops though (hard enough finding IL2)

Will I like this if I hated Oblivion?

Interesting! Thanks! I may actually give that a try.

I'm not sure, the gameplay is almost the same. Imagine Oblivion with a bit of Gothic atmosphere and you get the idea :)

Is nehrim standalone and playable with little or no experience with the previous titles of Arktwend and Myar Aranath, or does one need to play the first two to be able to get into the third(Nehrim?) in a proper way?

So has the unofficial expansion topic become the mod topic?
I remember the old level disks where they just grabbed a bunch of (I think public domain) levels and sold them for 5 bucks. Of course that's not really an expansion either, more a product of the age before the internet.
I don't think there were that many unofficial expansions sold.
I thought Hellfire for Diablo was one, but apparently that was an officially sanctioned third party expansion. And two for Starcraft which were also sanctioned, Insurrection and Retribution.
I did find this page while Googling, which even has Unofficial Expansion in the title.

That's pretty much all I can remember and find, but I'm sure there were more.

I think we're just discussing that with the tools and the internet freely available, modding games has taken the place of unofficial expansions, because now companies can't charge for stuff they didn't do, and companies don't permit others to charge for unofficial expansions.

On a side note, Gothic 3's community patch definitely qualifies as an oldschool third party expansion, since it fixes and adds to the game and only at the end of its production was approved by PiranhaBytes and given the source code for a proper finish.

I did some searching and (not really suprizing) came up with very little, these disks were basically just user made maps and campaigns that some company got permission to publish, I had a look through some of my old disks but couldnt find one as an example, you could try the planet AoE forums or heavens games section for AoE

Depends on how you define Gothic and how you define Oblivion.

If you mean a shallow game with laughable RPG elements then I'd say it's Oblivion.

I define them personally as such......Gothic Series(minus the first two's control schemes)=GOD TIER.......Oblivion(Vanilla)=SHIT TIER.

Modded Oblivion? Maybe Meh, iz s'okay tier.

Well, I found these:
Conquest of the Ages
and
Aztech's Armory Campaigns for Warcraft II

Made by the same guys. Searching through the Unofficial Add-Ons list there is a bit weird though, as they list Hellfire (non-canon but official) and Cryptic Passage Way for Blood.