If you like flying around shooting spitfires, messerschmitts, yakovlevs and mustangs and would like to do it for free (ie no game purcahse and no subs) then try out Air Attack. It's been going for about 5 years now and the 640x480 graphics may have been ahead of their time back then, but now will appear rather dated. However, for sheer online multiplayer carnage and a sprinkling of realism, it is still unbeatable. Squadrons exist and matches/leagues are run regularly, but your solo ventures into the "free for all" or "teams" arenas will be great fun.
Some of you may know it as the original Fighter Ace game, which was made by VR-1 software and still goes on today as FA3.5, albeit a subsription service. FA1.0 (called Air Attack for the UK market) was hosted on the original Wireplay (became Gameplay) service and then was lost due to licencing issues.
The game is packaged into an 8mb donwloadable file and the last remaining server in the world is located in Korea, where several hundred of us from around the world still play. The largest contingents are British, US, German and of course, Koreans. The average age of the average "pilot" is probably around 28/30 years old, which perhaps gives you some indication of it's target base.
If you fancy a blast, go to www.airattack.co.uk and hit the "get me started" button. It's reasonably straightforward, although the menu screens are in Korean so the step-by-step tutorial on how to get into the arenas is there for you as a guide. The forums are there too and you'll find the locals very welcoming and somewhat entertaining at times! Indeed, many have met up frequently and there is a very homely feel about it, where newcomers are welcomed with open arms.
Did I mention it's all free? Good luck if you decide to give it a whirl and if you see "Rocky" flying about, say hi, that's me.