You can pick up Total Annihilation, one of the PC's progenitor RTSs, for a mere $3.49.
In the late 90's, the PC was having a kind of RTS explosion, with classics like Command Conquer: Red Alert and StarCraft seeing release. Among them was Cavedog's Total Annihilation, which pioneered the "massive" RTS genre, focusing on gigantic-scale fights between massive armies. If you missed out on the original release, the good news is that the game is now available on Steam for a mere $3.49.
Total Annihilation, with its self-replicating mech armies, all-power "commander" unit, and breathtaking symphonic score, was the inspiration behind 2007's Supreme Commander. It has also inspired countless other RTSs since then, and you'll often hear people describe games as being "like Total Annihilation but with X."
The Steam re-release of the game is functionally identical to the Good Old Games release we saw earlier, adding nothing new to the game but simply reworking it to work on modern PCs.