Here at Spellhold Studios, we mainly support modding for three different engines/games; the Infinity Engine, Neverwinter Nights (the Aurora Engine) and The Elder Scrolls IV: Oblivion.
Modding for the Infinity Engine is now much simpler than in its early days, and now many people are creating modifications, new characters and new content for games such as the Baldur's Gate series, the Icewind Dale series and Planescape: Torment. SHS is host to a number of popular Baldur's Gate II mods, and is devoted to helping new modders get involved.
With the Neverwinter Nights Toolset available with the game, all manner of things are possible - additions, changes, and even completely new settings and stories. There are thousands of modules available to play, and thousands more in the works, as well as numerous additions such as new monsters, items, tilesets and rulesets to download.
The Elder Scrolls Construction Set gives you the power to create, modify and change content for TES IV: Oblivion. Mods can change many different aspects of the game, such as adjusting the visuals, interface and gameplay, as well as adding completely new content such as races, items, areas, armor and weapons.