Check out some maps from the Sin community. Please note the following meanings to the abbreviations:

SP - Single Player
DM - Death Match

These can be used in combination to indicate the map can be used as both single player and death match.

After downloading the zipped file and unzipping it to your hard drive simply make a folder called "maps" under the "base" folder in the directory that holds Sin. Import any .bsp, .pal, and .scr files to the maps folder. That's it! Then from within the game at the console, call the map. Whatever the name of the .bsp file is will be what you call when opening the map. i.e.: Lock.bsp is called from the console like this "map lock".

Please note that all maps submitted dated before 12/1/98 are made for the demo version of Sin.