There is still a good number of servers up. One of the most populated is Prolox but I usually don't go there: too many custom maps (that are usually camp-maps), custom perks (that unbalance the game), 2 entire minutes between waves and a max of 18 players (moreover it used to give 2500 start money, then it switched to 1200, now there should be 600).
If you want to try the mod as it should be try Geisterbahn: only standard maps (4), standard perks, 0:45 seconds between waves, 300 start money and a max of 6 players. It's very hard to win there but it's the only true KF experience I found online...
