I used to say that everything we build should be a platform except for games. What’s interesting is that games are becoming platforms in their own right. Dungeon Siege has the freely downloadable Siege Editor which allows you to “rework nearly every aspect of the gaming world, making Dungeon Siege not only a game, but also a platform for those who wish to create their own spells, dungeons, and even entire worlds.” There are several projects that do just that. And Dungeon Siege II is coming later this year, which looks amazing (all trailer graphics were rendered with the game engine). If RTS is more your speed, Relic – developers of Impossible Creatures – has their developer network which provides both a companion tool for enhancing Impossible Creatures as well as the Impossible Creatures SDK “which includes source code from the IC engine that can be used to create Total Conversion Mods for IC” (Relic Developer Network requires registration). Of course, there’s also the Allegiance source code which was released a few months ago.
I guess the new viewpoint is that everything we build should be a platform, including games.