Here’s several links to user created content ranging from guns to full on new maps, missions, and many other unique and interesting things. It’s all worth looking into and strictly for the PC. You cannot use anything here on the Xbox 360 or PS3.
Hopefully someone better fitted for starting actual topics for these migrate all the data listed in these links. Until there is a quick reference to all the links that have the user created content. Use at your own risk and always make sure to follow directions if they are given. If you are unsure what you are doing make sure you ask someone that does.
Link to fan creation thread in the old forums containing all of these threads and many others I may have missed.
http://forums.gearboxsoftware.com/forumdisplay.php?f=81
Here is a thread with internal links in it for the unofficial level editor. Use at your own risk!
http://forums.gearboxsoftware.com/showthread.php?t=88285
Here is a thread to Oasis. Oasis is needed for using any user created maps and whatnot. Follow directions thoroughly or else you won’t be able to use anything you download. There’s many creations within this thread. There is also a ton links to many things in here. Make sure to view them. Use at your own risk!
http://forums.gearboxsoftware.com/showthread.php?t=108800
Here is another thread with pictures and a huge discussion over user created guns and many other things. As well there are a couple tutorial videos going over the editor and how to create your own guns. Use at your own risk!
http://forums.gearboxsoftware.com/showthread.php?t=127716