One of my not-so-secret pleasures about some games is the ability to MOD them aka modify. It takes a lot of time, talent and a workhorse of a computer but the results are amazing. Unfortunately I haven't been able to create my own mod for years thanks to work.
Mod communities are a proverbial cesspit of creative and talented folks sharing game content for FREE...usually. Utilizing the limits of the game to add something new and provide MORE playtime with that game. If I find a mod community for a game I usually download a thing or two. (Or a million in the case with the Sims).
What makes a good Mod
Mods can vary in type from program fixes, hacks and cosmetic changes to full blown extra stories, missions or DLC.
One of my university courses was to create a Mod using the Neverwinter Nights 2 Toolset for game writing. The whole experience taught me what really goes into creating a good mod.
Pre-planning has a lot to do with it and learning is the other half. Planning and experience really are what make a Mod good.
Getting into Mods
Some games come with a developer like software to help you mod your game! Usually the PC version, I'm not all that familiar with console modding although I'm sure it's been done. Other games have level builders, such as Little Big Planet.
However, some games have communities surrounding them that use freeware or purchased software to create mods, such as the Sims.
Getting into Modding is helpful if you are interested in getting into game development. It can be a nice addition to a portfolio. Mods showcase your talent in game art, writing, programming and design.
Even being mostly a casual user of mods, I really enjoy and appreciate the effort. And I'm always glad to find someone has the same interest I had when playing the game and thinking "so-and-so would look better as a red-head" or "that ending sucked, I wish I could change it" or "I want to play the whole game naked". <---Probably most common mod ever. :D
Meh...just another fluff post.