Through my journey of gaming I've seen a lot of games come and go. Only a few of them made a large enough impression to be considered a milestone.
Shattered Galaxy - An old MMORTS that played like a command and conquer style game with all of the focus on the units instead of building. There was generally battles between 2 forces of up to around 32 players per side and in some special situations even more. Each player would have an average of 6 to 12 units on the battlefield with up to 48+ total units per player and they would fight over territory on each map. Battles where grand in size as hundreds of units fought across the map with an ever shifting battle line. Sadly it's been slowly dying as it was big back in the 2,000's and about 90% of the content was slowly removed due to "balance" reasons, aka people complaining. I miss those days and nothing has quite captured that game except one to my knowledge which is lower down.
Risk Your Life - This was an old Korean mmorpg that was probably a wow clone. I joined a server where the level rates where really low so most people stopped around level 40 out of 95 and did all the pvping there (a level difference of 5 would mean a HUGE difference in power). I've managed to get one character up to 92 and had some decent gear as well, it was a healer and one of the main things I remember was that I was high enough leveled and tanky enough that I couldn't be one shot by those damned invisible back stabbing assassins though I couldn't kill them in a 1v1 because I only had one lockdown spell and not much damage to back it up. It was awesome being attacked by 5 players level 70~85 by myself and being able to stale mate them until I finally gave up and teleported away. It was a shame though that whoever made the game forced all the community servers to shut down and the sequel RYL2 didn't have the same spark to it with their changed mechanics.
Tom Clancy's Endgame - This one probably hurt the most as it was the closest to Shattered Galaxy though not on a more skirmish scale. The bad matchmaking system and the problem with losing unit progress on their death caused this game's early death. It was great participating in 2v2's and 4v4's as I didn't like going 1v1. I liked the teamwork more than playing by myself.
Chromehounds - This is probably the biggest shame out of them all. This game was magical and I sank in hours and hours into it, maybe even more than Shattered Galaxy. The customization and slower combat was awesome, it was like the thinking man's shooter as you could win or lose over the small and big decisions in and out of battle instead of being who had the best twitch reflexes. Lucky for me there is a spiritual successor to this game called M.A.V. that has popped up by Bombdogstudios. It's still in Alpha but the developer isn't one of those half finished people who'd drop it midway through and the game play is already there. I've had tons of fun skimming people on the server's I've been hosting for M.A.V. during the Friday Night Fights we've been doing almost weekly.
I'm not going to put stuff like UT2k4 and CoD:MW and MW2 on here as I've grown apart from cod and UT has it's new game being constantly developed but they where pretty awesome while they lasted too. Shame what cod became.