well, there really are two types, there are total conversions, which change EVERYTHING in your game and both clients and servers have to be running it, a good one is desert combat which can be found here:
www.desertcombat.com which takes place in modern times and right now (it's only alpha .2) is between the american army and iraq, i assure you, it's rather fun. another good TC is Soldier 9's Realism Mod, found here:
http://s9rm.nucleardays.com , this mod makes bf42 more realistic, you die faster etc... they are also planning to release a new version soon. i really like this mod but there's never a server up =/
okay, onto the mini-mods such as merciless, now this changes some stuff in your game, it's only client side so you can still play at any server you see fit... the website is:
http://www.planetdungeonsiege.com/me...ctions/bf1942/
and it really really makes your game a whole lot better... it's also well worth it to download all of their texture packs, ie: operation market garden, bocage, and battle of the bulge for they make your game look a whole lot nicer...
and yea, that's about all i have to say for now...