what mods that are so big that it feels like its own game or it feels like a sequal/expansion pack of said game.

I have one

Hyrule: Total War:

http://www.youtube.com/watch?v=vfhy7B8mSlM


Oh INB4 DayZ

Nehrim: At Fate's edge

There's much more ... And I'll update this post as I remember them :D

.....wat......
nice

The Nameless Mod for Deus Ex is all sorts of fantastic. Oozes polish and love, and full of what makes Deus Ex great. Quite long, too.

Garry's Mod
Counterstrike 1.6
Killing Floor

/trollface

1.13 and derivatives. Did you know that quite a few of the UB Campaigns are currently being ported to 1.13? Swing by The Bear's Pit for more info on the project.

Pool of Radiance for Neverwinter Nights. There'a also PoR for NWN2, but I haven't pöayed that.
http://nwvault.ign.com/View.php?view=modules.detail&id=1706
http://nwvault.ign.com/View.php?view=nwn2modulesenglish.detail&id=63

Ultima IV reborn for Neverwinter Nights:
http://nwvault.ign.com/View.php?view=modules.detail&id=2693

Then there's also Ultima 6 and Ultima 5 remakes for Dungeon Siege, but I personally feel that the DS engine does those games a disservice.
http://www.u5lazarus.com/
http://u6project.com/

Do they have something similar to morrowind and skyrim not eactly nehrim but something like what nehrim is like a user created continant

Well, Morrowind's got Tamriel Rebuilt, which is half done (like... Half of the continent is 100% done, the other half isn't, so you can acually play it until they finish another bit - and yea, it's quite huge and yes, they're still actively working on it.

And Skyrim's got Moonpath to Elsewyr
Fall from Heaven 2 for Civ IV That's not exactly it's own game tho, is it? It's still the same game with a lot of new content and features.

The Dark Mod for sure.

Black Mesa Source has new screenshots and says its "pretty close to completion"

http://www.pcgamer.com/2012/08/29/black-mesa-source-has-new-screenshots-is-pretty-close-to-completion/

Do I need Half Life 2 to get this mod? or is it a stand alone game?

Well they say that for quite a while :D

Depends. If it ends up using HL2 assets, you'll need HL2, if not, it'll be standalone / it'll work if you only have TF2 downloaded (it's free)

Not sure, does a WW2-era multiplayer match play the same as a 90s singleplayer campaign? Does a zombie apocalypse in Arulco count as the same as a liberation effort? Does SOG '69 count as a "Banana Republic", even though it takes place in vietnam?
While you may say that the underlying game is still the same, that is like saying any killfest fps is the same as any other, eg Serious Sam and Painkiller.

Wait really?

*lloks at like*

Well, that's exciting news! Not completely convinced that it will be for real this time, but I still would like to re-visit the old Black Mesa adventure with some shiny new visuals.

I thought 1.13 just updates the base JA2
It would, they also say that like 10 times already. It's like me when crossing a deadline.