Metal Fatigue. Old 2001 RTS by Psygnosis and includes, aside from the standard RTS fare of tanks, buildings, and upgrades, maps with three playable layers and giant customizable robots! You can choose what kind of legs and torso the 'mech will have, along with the two arms it will use. For example, it can have a laser katana (a katana with a laser blade, I s#!t you not), arm blades (like Wolverine's claws), a cloaking torso, and HTH Upgrade legs (doubles melee damage). If a Combot, as they're called in the game, is equipped with bladed weapons, then it has a chance to actually dismember the enemy 'mech. These disembodied parts can then be recovered and reverse engineered so that you can use them yourself!
Also, there's Warzone 2100 from Pumpkin Studios. Post apocalyptic setting where your tasked with recovering 'Pre-Collapse' technologies to use in the advancement and survival of the human race. The gimmick of Warzone: Customizable units. You can choose the weapon, propulsion, and main body of your units and produce as many as you want. Actually, every unit you produce is of your own name and design (except Cyborgs and the starting Truck, but the Truck is still customizable.). At this current point in time, Warzone 2100 is active again, and free to download from the new developers' website: http://wz2100.net/