Open gaming allows you to benefit from what everyone else in the open gaming community is doing, and it makes it easier for you to contribute back to the community.
An open game allows you, or your gaming group, to take the actual text of the game, modify it, and then make your modifications available to the rest of your group or even to the rest of the world. Someone else can then take your version of the game and use it, if they like it better than the other versions that are available. Their enhancements, if they decide to publish, are also available to the gaming community as open.
Let’s say that your gaming group has a handful of “house rules” that you use. With an open game, you can take the text of the game, insert your house rules, and make the new version available to all of your group’s members without any hint of breaking copyright law--as long as your version is also open.
You can use house rules in any game, open or closed. But an open game makes it a lot easier to distribute your house rules in a readable form. An open game must always provide the game in a form that makes it easy for you to add house rules directly to the game.
Everyone uses house rules. What you’re really saying is that you don’t keep track of your house rules. No game has rules to cover every situation, and your group, like any, makes rules up on occasion. Even if you don’t write those rules down, you still benefit from open gaming, because other groups will. Some of them will make enough house rules that they decide to publish their version of the game, and you can choose to use (or not use) their new version. Or you can pick and choose from their new rules.
Then play it. But which version? First edition? The Blue Book? The Original? Second edition? Third? Or did you switch to it after discovering it difficult to get new copies of the game you preferred, but that their company bought and then buried?
Games come and go, especially closed games. Open games are required to be available in a freely-editable format. Your favorite version is on your hard drive, ready for reprinting for new group members at any time.
So don’t make your game world open. You can still play an open game and create closed supplementary materials, just as you can make accessories for closed games and non-games without permission from their respective copyright owners. People make “add-ons” for Gibson guitars, Harley-Davison motorcycles, and Kitchen-Aid appliances all the time, with no license needed. As long as you don’t infringe on the copyright of the open game, you’re free to do whatever you wish. Of course, the open gaming community would like your add-ons to be released openly, but that’s your choice, not ours, and has nothing to do with whether the game is open or not.
If you do decide to make your add-on open, however, you can take advantage of any other open material under the same license, and use it within your accessory. You can copy, paste, and modify to whatever extent is needed to make your accessory the best accessory possible.
And even if you choose to continue playing a closed game but the idea of open games appeals to you, you can just as easily make an open add-on for a closed game.
If you’re modifying an already open work, you usually have no choice: you must use the license that grants you the right to publish modified copies of the existing work. If you are releasing your own new work, read the information at the Free Roleplaying Community web site. I like the GNU Free Documentation License (http://www.gnu.org/copyleft/fdl.html); it is well-established, and widely respected as an open license. Gods & Monsters, for example, is released under the GNU FDL, as is this document. If you publish any versions of Gods & Monsters, including this document, you’ll need to abide by the terms of the GNU FDL.
If you choose a license other than the GNU FDL, be wary of any license which restricts what you can write. There is at least one ‘open’ game license which forbids mentioning what games the work is compatible with, and forbids discussion of other works.