An update for Left 4 Dead has eliminated any hacks that cheaters were using within the online modes. Reports came from people that people were using hacks to create giant-sized players and super-enemies along with the spawning of tanks in huge numbers in various sections of the levels.
The problem sounds much like the days of hacking on Halo 2 for the Xbox, but Valve who make Left 4 Dead has released a title update which will fix all the problems.