Hello hell-bringer,
Henjo's Diablo Page is the place for getting mods. It has the most extensive list of mods on the net IIRC. Although for some mods, he doesn't have the latest version.
Most of the better mods are listed on the LL's Mod page that SomethingDutch already gave the link too. I've played Absymal, Dr. Zed's mod, Hellfire: The Dark, and Hellfire Fixed, and the best one imo is
Hellfire: The Dark. As the name suggests, its only for Hellfire, so you might not be able to play it. Otherwise, I suggest you try out Dr. Zed's mod(DZM). It only works for version 1.07(along with practically every mod for classic diablo), so you will need to have a separate install set up for mod playing. If you can get Hellfire though, HF:TD is the way to go. Its the only mod that is played regularly online anymore and can be really fun with some friends. Note that I'm talking about a group of about ~10-15 people max.
Creating your own mod is a very difficult task I'm told. It involves hex edditing and a bunch of other crazy, strange stuff. There is a mod making forum up
here, as well as a hex edditing workshop
here. Henjo's Diablo Page also has a bunch of mod making tools.
(EDIT: Edited out link to Henjo; it required copy-pasting. I don't link to Henjo from main page to keep the newbies hands' "clean," or so to speak. Interested parties can google for Henjo if sufficiently curious. -
Lem)