Kotor 1:
 Redhawke made some planet and new area mods: Ord Mandell (
http://pcgamemods.com/7699) and Weebul and Utiinii's (
http://pcgamemods.com/8012).  
 I made new areas too but it was a joke mod: 
http://pcgamemods.com/u/3451/) (if you don't know the people at Holowan it will probably be quite weird). 
 Not a new area but definitely worth it, Tanesh's Mekel recruitment mod is the best recruit mod made until now: 
http://www.starwarsknights.com/kotor1mods.php) (all the mods on this page are good) 
 Xavier2 also made mods based on the movies if you like it:  
 Some other goodies for your override folder which add to the storyline (some material got cut form k1 too):
 - Bastila on Korriban : 
http://pcgamemods.com/5374/)
 - Deadeye Duncan on Manaan: 
http://pcgamemods.com/5410/)
 - DS female alternate ending: 
http://pcgamemods.com/5326/)  
 hehe and a small goodie I always use when I play DS: 
http://pcgamemods.com/8708/) (got to plug my stuff somewhere :p) 
 If you look at the mods at pcgm you'll find much more.  
 As for Kotor 2, there is GSccc's Rise of the Sith Demo (
http://pcgamemods.com/15029/) (I didn't tried it due to file size and time issues) and the USM mod: 
http://www.lucasforums.com/showthread.php?t=149509) (It's new item but it also fixes a number of issues in the game and all sabers have been integrated in the storyline)  
 Moving this to the Modding requests forum ;)