http://www.gamebanshee.com/morrowind/locations.php
This is a pretty good site, but
www.morrowindfiles.com is where i usually get my plug ins, although lately the site is down very often.
Some mods can make the game easier, but others make it tougher to beat your opponents, like the one where you down download new kinds of monsters, and even flying firebreathing dragons.
Some mods can cause the game to crash, others can come into conflict with eachother somehow, so i'd suggest you take one at a time, and not a whole bunch, otherwise you'll be spending more time figuring out what's wrong, than you'll be playing Morrowind.