The stocks with the battery storage are the VLTOR's but there are other gee whiz stocks too, optics are EOTechs, Aimpoints, and ACOG's (any of which are awesome), there are a number of new items too. A few of which are the Tango Down Battlegrip, and vertical grip. There are Vert grips by KAC, ARMS, Dieter, YHM, etc, etc. There are railed forends from, ARMS, KAC, Daniel Defense, YHM, LMT, Troy, etc. There are Tac Lights like those from Streamlight and Surefire. There are BUIS in the form of fixed and foldable, front and rear from ARMS, KAC, Troy, LMT, YHM, etc. There's also NV, lasers, IR, etc.....
In short there are about a zillion different ways to mod out your weapon. There is no real standard way to trick it out. Some of the products I mentioned are issued to SOME troops, some are not. Some are privately purchased and end up in the Sand Box, some are not. People will argue that this is mil-spec and this isn't, and vice versa. Its all really just about what you like best and what works best for you.
-Hershey
P.S. nearly forgot the stuff by First Samco and GG&G. There I think that covers most of the big companies!