There aren't really any flight sims online. You would need to buy FS2004 or FSX if you want to play with other people (you need to download extra software if you want to connect to the big networks). If you only want single player, there are also a few other smaller sims around, like X-Plane and Flight Gear (which is free) to name a few. I think, however, in terms of the number of players (and therefore both freeware and payware addons), the Microsoft sims are the best. However, you will need a good computer to play them.
