xMule is a fork of LMule, and was the defacto eMule client for Linux until aMule appeared and challenged that position. However, xMule is NOT under active development, primarily by Avi, as of 2009-01-18. Please refer to the xMule website for more information on its current state.
aMule is a fork of xMule, though both projects have moved further and further apart since then. Diplomatic relations between the two projects are sadly in a rather sorry state.