The Counter Strike game has always had a massive collection of modders to make your servers different and unique. With wide a range of things like administration scripts, game type modifiers, protection scripts etc. it is very hard to look at every one, but we will look at some of the most common and popular. This should help you get to grips with running mods on Counter-Strike Source servers.
Mani Admin Plugin
Arguably the most popular mod on CSS and Team Fortress 2 servers. Mani Admin is a very popular choice for server administration which is written in C++ and one of two of the top admin mods. The plug-in features all the day to day admin stuff like banning, kick, map changes etc. If enabled the mod supports non-admin initiated voting. The plug-in also has support for custom admin skins as well as the custom sounds like Quake. This mod is great but for many years has been the target of exploits by hackers due to several known bugs in it.
Metamod is used as a layer between the engine and the game and allows the plug-in to interact with the commands that go between the layers. This is just a base to allow other modders to build there plug-ins from. This as well is a C++ plug-in and has been designed to not have unnecessary functions like MySQL or game play modifications.
There are plenty of game servers that use Sourcemod as a modification . This is another administration plug-in that requires the use of Metamod to run. The mod has the usual plethora of tools that any admin requires but also has great admin permission system. With reserved slots it means that you and your mates can be guaranteed a space on your server without the need to wait. Player voting can be performed just like Mani. Sourcemod can be expanded with over 450 community submitted add-ons to it. I have always been the bigger fan of Sourcemod over Mani as its more robust and a great interface.
Eventscripts has been designed to add extra functions by writing your own scripts of use one of the hundreds that members of the community have created. Eventscripts was made so that the language was easy for everyone to use and learn. Eventscripts will even allow you to make use of the Python scripting language.
This is an extremely popular add-on for Sourcemod that makes the game into a level system. Starting off with low power weapons like pistols, you advance through to higher levels by killing the opposition and the last of the levels require the most skill with the likes of HE grenades and then a knife. This is just one of the popular add-ons for Sourcemod and works well with CSS Deathmatch.
This mod turns CSS into a HL2 style Deathmatch game allowing either team or free for all games. You get spawned to a random part of each map after death and get your weapons by selecting from a menu.