An OGame clone is a game that shares similarities with OGame (either in terms of gameplay, aestethic, or even as a general game idea). It is most often made by other publishers, unrelated to Gameforge. These games are hosted on private servers, where you can also find Clone builders, where all the other clones are based from, like 2Moons, XOrbit or Galaxy of Masters. The package is distributed free. It is OpenSource and is based on PHP and MySQL or .NET and MYSQL.
A list of OGame clones can be found at gametoor.com
Calypso Universe is very stable Ogame clone game with probabilistic battle engine, great and useful modifications and big and active player base: http://calypsouniverse.com/
http://stellaruniverse.net is a new game, If you want to get a chance to be a leader then join Stellar Universe.