Suggestion:
>Counter-Strike Source:
/// Surf maps
/// ZombieMods
/// Or just host the normal server with ranks and bank. Believe me, it works.
>Garry's Mod:
/// ZombieMod ( Similar to Noxiousnet server )
>Zombie Panic Source: ( I'm sure you guys know this already by now, zombie modded server are very popular beside the surf map. ) < This mod is free if you owns HL2. >
>Battlefield 2/2142/BC/BC2 servers
>Left 4 Dead: Now i know you hosted this before, but I'm not sure about the people coming in the server often.
>Left 4 Dead 2: Sure, it's still popular, but to speak, i think it's a money wasting thing here.
at last, why don't you think of hosting servers for COD4:MW and MW2 ?

Lately, I have been playing on OpenTTD and Alliance of Valiant Arms (the free game from ijji.com), and usually playing on TF2 and HL2DM as well.