To play with alternate costumes, press ~ to bring up the console and type any of the following codes. Note: These cannot be activated by the host server.

bind w "+forward;model sas"
bind d "+moveright;model gig-n"
bind a "+moveleft;model l337"
bind s "+back;model arctic"
bind uparrow "+forward;model vip"
bind rightarrow "+moveright;model sciencetist"
bind leftarrow "+moveleft;model gorila"
bind downarrow "+back;model sas"