MPlayer2 was forked months ago from MPlayer and in a matter of weeks has gone from its first beta to a release candidate and is now onto its second release candidate. These releases are certainly far more frequent than the original MPlayer project, which easily can go a year before seeing a new development release, thus leaving many users to depend upon snapshots from the MPlayer VCS. MPlayer2 is largely the work of Uoti Urpala, who last year was basically denounced as a member and developer for the MPlayer project due to "long standing differences" with the MPlayer Team, as said in this email.
Over the current MPlayer media player, MPlayer2 has support for ordered video chapters, dynamically loads system libraries (including FFmpeg and libass), support for precise seeks, uses the libass library for rendering sub-titles by default, better support for NVIDIA VDPAU, and has better pause handling support.
MPlayer2 is also designed to be more performance friendly by using the multi-threaded version of FFmpeg. It also gets rid of MEncoder and the internal GMPlayer GUI. Other differences are talked about on this comparison page.
MPlayer2 is being developed as version 2.0 right now. MPlayer2 2.0 RC2 can be found at MPlayer2.org. There's also this independent blog posting from this morning about experimental MPlayer2 packages for Fedora.