One of the most famous user created mods for an already established game? That's right, your favorite squad based FPS Counter-Strike was originally a user based mod for Valve's original Half Life. This in the modding world is what is called a total conversion, meaning that the user created content is completely stand alone from the original product. In the case of Counter-Strike, the original engine became free software so the Half Life community was able to create an entirely different game using it. Another famous team based FPS mod? Team Fortress was a mod based off of the Quake engine. This gave way to Team Fortress Classic and Valve's highly successful sequel Team Fortress 2.
Another category in modding would be partial conversion. Instead of fully converting the original product into a completely new game, modders change little things like textures and models up to creating new maps and play modes. Defense of the Ancients (DOTA) was created as a mod for Blizzard's Warcraft 3. This mod became such a hit that it gave rise to a new video game genre, Multiplayer Online Battle Arena (MOBA). And how was DOTA made? By using the toolset that Blizzard provided with the release of Warcraft 3. Many developers package mod making tools with their games. Examples of this include: The Elder Scrolls Construction Set which was shipped with The Elder Scrolls III: Morrowind, the Aurora toolset included with Neverwinter Nights, the Obsidian toolset for Neverwinter Nights 2, and the toolset included with Starcraft 2. Another toolset that has a massive community base would be Garry's Mod which is described as a "standalone game" but really, it's modding tools for Valve's Source engine.
But isn't modding just for PC games, you ask? Previously, yes. Unquestionably, there was little to no way to mod console games short of putting mod chips into consoles (a highly illegal act as well) But then the current gen consoles came to existence with their hard drives, SD card slots and cable/wireless connections. Game developers like Bungie started to include map and content editors right on the disc with the introduction of Forge with Halo 3. The community of certain games have been jumping into console game modding as well. Project M, a user created mod for Super Smash Bros. Brawl, alters the gameplay to be more like the previous iteration in the series, Melee, as that style was more popular with the Smash Bros. community (because everyone hates tripping)
The ability to mod games extends the life of those games far beyond their expected lifespan. It also spawns communities that make creative content and push a game's capabilities past that of the original developer's expectations. So next time you beat that game that you love, look online for some mods and see how much more game you can get out of it. I'm going to leave you with one of my favorite mods ever, but feel free to share some of your favorite mods in our comments section.