There is a private server for old Westwood Studios games (mostly Command & Conquer), which allows you to play online on a much more stable and reliable server than the XWIS servers (the company that took over the multiplayer after Westwood's servers shut down).
The following games are being supported by CnCNet:
Dune 2000 (Legit free download inlcuded)
C&C Tiberian Dawn + bonus missions (Legit free download available)
C&C Red Alert 1 + both expansions (Legit free download available)
C&C Tiberian Sun + Firestorm expansion (Legit free download available)
C&C Red Alert 2 + Yuri's Revenge expansion (These are not legimately free, you need to own them in order to play them on CnCNet)
CnCNet also features the following mods:
Mental Omega (Yuri's Revenge Mod)
Dawn of the Tiberium Age (Tiberian Dawn + Red Alert 1 on Tiberian Sun's engine, standalone game)
C&C Reloaded (Yuri's Revenge Mod)
Twisted Insurrection (Standalone game based on Tiberian Sun)
CncNet has a fairly active population for most of the supported games, at EU evenings, there's around 500 people online in total.
Link:
Note: I am not affiliated with CnCNet, I am just a player who felt like promoting their service.
