In the first Killing Floor, you can easily add mutators from a Mutator Menu available when you're setting up your match. When you are joining someone else's game, the mutators in use will be listed in the info on the bottom-left of your screen.
If you join a server running a mutator you don't have, it will be temporarily downloaded before you join. You can permanently acquire mutators to add to your list by downloading them from the Steam Workshop or certain websites.
When browsing servers, a symbol to the left of the server name will tell you whether or not Stats are Enabled (ie. you can progress your perks and earn achievements). You can also specify for the browser to not show you servers where stats are disabled.
Official Killing Floor Mutators
These mutators are available in the first Killing Floor and will not prevent Perk Progression:
- KillingFloorMut: This is the default mode
- No Karma Decorations: Removes Karma Decorations from the map
- No Trader: The Trader never opens between waves.
These mutators are available by default, but will prevent Perk Progression: