Mechwarrior 4 Mercenaries' MekPak 3 from Mektek:

http://www.mektek.net/joomla/index.php

The NBT Hardcore mod is also quite good in a more traditional BattleTech way, and can be installed side by side with MekPak 3:

http://www.netbattletech.com/nbt-hc/

Both mods are still being updated and played on a regular basis.