Absolute classic game and one of the best RTS games ever. Only for 4.99 USD as part of the steam mid week sales (Normally $19.99 which is still worth the price imo). I haven't seen it posted yet so i thought i would do my duty.
Alternatively the 4 pack is only $15 USD
The steam version has updated textures and general improvements and is fully integrated with steamworks (including steam workshops for custom maps, etc)
One of the greatest RTS games