What is eMule ?
Emule is one of the biggest and most reliable peer-to-peer file sharing clients around the world. In May 13th, 2002 a guy called Merkur was dissatisfied with the original eDonkey2000 client and was convinced he could do better. So he did. He gathered other developers around him, and eMule Project was born. Their aim was to put the client back on track where eDonkey2000 had been famous before, adding tons of new features and a nice GUI. They couldn't imagine what impact this decision would have...
It has gained its' success because it is free software and many developers are able to contribute to the project, making the network more efficient with each release.
NOTE: aMule is a separate project and is not related to eMule (apart from being originally based on it and having a friendly relationship).