From SA-MP Wiki
|Win this gamemode by being the player with the most money, obtained by killing other players. A popular gamemode on many servers due to its lack of any 'real' objective, making it a freeroam mode|
|Rivershell||A water based CTF gamemode, where your team must win by capturing a slow moving boat from the opposing team's base before they capture your team's boat.|
|RC Barnstorm||This is a DM gamemode. When you spawn, you are placed in a RC Baron. There is no goal in this gamemode, just shoot the other RC planes.|
|Grand Larceny||This is a Freeroam gamemode where you are free to go wherever you want. This gamemode was first used in the 0.3a public beta test.|
Creating gamemodes can be anything from simply placing coordinates into the Pawno editor and compiling to creating fully-fledged missions with progressing spawnpoints, various objectives, and custom ingame commands. Some basic examples of how to script can be found here.
Here are only a few of the released game mode scripts, the best place to check for new game mode scripts is here.