Sigma Storm 2 - The game we are talking about ;)
Sylen Heaven - A Blog by Sylen with huge resources about missions, ressources and components.
Red Cells Forum - Our Forum
Red Cells Wiki entry - What the name suggests ;)
HCS' official Ultimate Sigmastorm 2 Guide - An item list, maps list, entity list, ...

Conquerer7's maps from L005 - L140 - Standard map for lower levels. You will need this!
EdTheHeads maps from L150 - ... - A good map for the levels from L150 and up. You will need this!