An OGame clone is a game that shares similarities with OGame (either in terms of gameplay, aesthetic, 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.
One of the first clones ever built was made by an Argentinian that used the nickname "Perberos", from there a French team continued his work and built XNova.
XG Proyect continued in 2008 XNova's work and built the first Spanish speaking community, to be positioned as the top Open-source OGame clone, mainly focused on bug fixing and making the game as much similar as the original game. In subsequent years XG Proyect lead to other projects like 2Moons.
The majority of clones and newer projects are based on these Open-source projects, mostly built on PHP, JavaScript and MySQL.
Some clones offer many unique features that even OGame does not have, or features that were developed by the clones way ahead of OGame. Some of them like the Auctioneer, Scrap Merchant, Academy skills, extended Officers and Governors, and even gathering or distributing resources and fleet across all planets and moons, substantially helping players manage their empires. However, most servers do not provide support to their players, and the projects are abandoned to their luck.
A list of OGame clones can be found at ogame-private-server-list.info and private-server.ws/top100/OGame
According to players votes, as of 2019, the best OGame inspired game is niburuspace.com. It's the most popular and recommended private server due to its stability, daily support to players, multiple languages, unique features and free play.