http://www.gamesas.com/bgsforums/index.php?showtopic=1033691&hl=skyrim, one of the few released mods of this kind (an awesome mod by the way!).
and there is also another released http://www.tesnexus.com/downloads/file.php?id=16657 that might be a bit less popular.
http://www.gamesas.com/bgsforums/index.php?showtopic=982779&hl=skyrim, which is still a WIP but very promising ^_^
and another WIP for the same region is http://tesnexus.com/downloads/file.php?id=23636
http://www.gamesas.com/bgsforums/index.php?showtopic=1030644&hl=skyrim still a WIP and as above: looks great so far!
This is one of the first http://www.tesnexus.com/downloads/file.php?id=18715 mods, but there are two more projects going on at present: http://hammerfellmod.ning.com/ and http://www.gamesas.com/bgsforums/index.php?showtopic=1054531&hl=morrowind, these are both great WIPs
http://www.gamesas.com/bgsforums/index.php?showtopic=1023007 is another awesome WIP
Then we have http://tesnexus.com/downloads/file.php?id=28908 and http://www.tesnexus.com/downloads/file.php?id=22555, which are two nice WIPs based on the same Tamriel Heightmap used for Skyrim Improved and Blackmarsh Improved.
Also Silgrad Tower has been working on a mod entitled "Valenwood" for a while now, here you have the link to their forum thread: http://www.silgrad.com/wbb2/thread.php?threadid=11350
By the way, I have just realized that the amazing mod entitled "Abecean Islands" will be included in the Valenwood mod by Silgrad Tower.
For the Summerset region there are also these 2 projects:
http://www.gamesas.com/bgsforums/index.php?showtopic=1080354 and
http://thesummersetisles.weebly.com/, however this one looks dead to me.
The province of http://highrock.weebly.com/index.html is another of those WIPs I can't wait for!
and here is the http://www.gamesas.com/bgsforums/index.php?showtopic=1073237 for the same mod.
And http://www.gamesas.com/?showtopic=1080432&st=0&p=15739167entry15739167 by Onra is a huge project with a single heightmap for all provinces in Tamriel!
Then there is a series of mods who tried to recreate parts of Morrowind, these are:
http://www.tesnexus.com/downloads/file.php?id=16976, which I think is a dead project by now.
http://www.gamesas.com/bgsforums/index.php?showtopic=1060544&hl=morrowind another great WIP
http://www.gamesas.com/bgsforums/index.php?showtopic=1058939&hl=morrowind, which recreates a region of Morrowind.
And last but not least http://sites.google.com/site/ampsoblivionmodpage/bittercoast---an-oblivion-mod, which looks really nice.
Are there any other promising mods out there that I am missing??