The Authentic Colonization mod for Civilization IV: Colonization does a pretty good job of improving the game's usability and generally improving the game. I am looking forward to trying out Religion & Revolution v2.1 when it releases, as it is built upon TAC.

DOOM 64 EX really improves DOOM 64, by allowing the player to use mouse and keyboard controls, and to customize other bits of the game. Ditto with GZDoom. I love playing classic FPS games with modernized control schemes, simply because they have much more interesting levels and fights when compared to most modern FPS games.

The New Dark patches for Thief, Thief II, and System Shock 2 are awesome. They improve the graphics, make the games compatible with modern hardware, and generally make them more playable.

Cool I'll check it, even if such projects are worth for me only if they respect the original lore+look&feel, are mostly bug-free, and add very good content.

Now that I'm remember, the DVD edition of Spelforce 2 Gold crash continously if you not install a comunity path.

I own that. The big problem was that half of them shipped with incomplete install codes. :P

Well, it seems to me that the RP modders were/are very serious about and dedicated to respecting the lore and the original designs, but in some cases the official docs only had rough sketches, requiring more extrapolation to make something that could be put in a game. I'd certainly recommend trying it, after at least one proper playthrough of the vanilla game (that is, using only the unofficial patch) -- for one thing, that makes it easier to judge how well the new content fits.

Ok, but those titles were great from the start, those are not game-changing fixes.

When your games stop working on modern hardware, such patches and source ports allow the games to live on. I think that is really important, because I don't want my games to become a thing of nostalgic memory.

But then we'd need to list all games on GOG!
I'm mostly trying to identify previously bad\average titles or bugged good\great games that have been fixed\improved and are worth playing now.
Let's focus on these as much as possible please.

X3.

Is it better than WBC2 with\without the mod? (I bought that)
Some reviewers just hated the third..
(where are W2 and W3dr instead?)

Well, the classic example has already been mentioned. Vampire: The Masquerade - Bloodlines has been made playable by fan-patches and is among the best RPGs out there.

But if total conversions count too, there are some more mods to look out for:
Dungeon Siege 1 has the total conversions Ultima 5 - Lazarus and The Ultima 6 Project - both vastly superiour to the original game.
And for Oblivion there is the total conversion Nehrim (a mod for which by now countless add-ons exist), which also is much better than the original. It just took the graphics engine of Oblivion and created a vivid original world with an interesting storyline (a bit like Gothic 'feel' meets the graphics of Oblivion)

The whole series really didn't change much over the years. In my case I like WBC3 marginally better, but it really is a matter of preference.
-Same sprites reused in each game for recurring units. Sharper images, but otherwise identical.

-Each installment adds more stuff: more races, more hero classes, more abilities.

-The campaigns were the most significant difference between games. The first had a plot that mandated the use of a particular race. The second had no plot, just a large "conquer the world" style map that you could play any race in. The third also let you play any race and had a pretty decent plot, but the story was clearly written with "good" races in mind and made very little sense playing -for example- demons.

-The third also had a different hero building system. IIRC it was much more streamlined and easier to work with, but might've had fewer options. Still a more complete system than you'll find in any other RPG/RTS hybrid.

-The WBC3 fan patch alters stats on a lot of units and makes other changes like adding timers to summoned units (they used to be permanent). I feel like the changes probably make the game a lot better if you play it as a multiplayer RTS, but it nerfs a lot of the RPG-ish power building over time that I loved the game for. This patch comes preinstalled on the GOG version, so if you wanted to play it vanilla you'd have to hunt down a boxed copy or find some way to undo the patch.

Still?

Thank you +1
However, are the races and heroes still unbalanced and not that different? (read in a review)

Dark Souls - unplayable on PC without the Durante fix.