I've lately been playing a lot of games in the Command & Conquer RTS series. For those of you that weren't old enough to pick up a keyboard yet, Command & Conquer was a staple series back in the 1990s, created by Westwood Studios, which alongside Blizzard Entertainment pretty much set the bar for real-time strategy. At this current point Command & Conquer (colloquially known as just C&C) is split up into a few universes:
Tiberium: The first Command & Conquer was set in this universe, which revolves around a struggle between a United Nations special operations group known as the Global Defense Initiative (GDI) and a terrorist militia known as the Brotherhood of Nod (NOD) over the span of several years, with the inclusion of a dangerous element (Tiberium) that you also collect as a resource. Command & Conquer was set around the "present" era of 1995 and then later on moved onto the years 2030 in the sequel Command & Conquer: Tiberian Sun, 2047 in Command & Conquer 3: Tiberium Wars and finally 2062 in Command & Conquer 4: Tiberian Twilight.
Red Alert: An alternate reality universe revolving around an alternate World War II created by the actions of Professor Albert Einstein, travelling back in time from 1946 to 1924 to take Hitler out of history as he left Landsberg Prison. With Nazi Germany never rising, this allowed the USSR to grow and attempt to take over Europe in Hitler's stead. After the defeat of the Soviets by the Allies and a period of seeming peace, the Soviets would this time strike against America in Red Alert 2 in the 1970s, only to have their top psychic adviser Yuri stab them in the back to form his own faction. With the Soviets' second defeat and Yuri's own defeat, they had made a desperate gambit to travel back to 1927 to assassinate Einstein, who gave the Allies the technological edge in the last two wars. This however would end up creating a new timeline without nuclear weapons and also one with a third player in worldwide affairs: The Empire of the Rising Sun (Japan).
Generals: An offshoot that isn't linked to either Tiberium or Red Alert, Generals gives you control of either the U.S, China or a terrorist group known as the Global Liberation Army. The game follows more traditional RTS standards like what you'd find in Age of Empires.
Even though the official EA servers for some of the games are no longer in use (Red Alert, Red Alert 2/Yuri's Revenge, C&C, C&C Tiberian Sun), a community group known as Strike Team was given permission to re-raise the master servers for the older games, and there's also alternatives like CNC-Net. Also if it's to your interest, Electronic Arts released Command & Conquer, Command & Conquer Tiberian Sun and Command & Conquer Red Alert as freeware, downloadable at several sources on the web.
If you want to do a multiplayer game, or if you need help with setting one of these games up, please feel free to let me know and I'll see what I can do to help. One of my ambitions here is to see EGC expand to create a Command & Conquer group.