French coders BlackDevsTeam (composed of Darkside, Zer01ne, and Spimort) have released an alpha version for their 0x584d42 project. Whether you want to call this a PSP shell or an XMB replacement, then so be it. For the record though, the BlackDevsTeam calls it an XMB alternative, much like Auraomega's Project4.
Note that this version is still in alpha, and was released to the public more as a proof-of-concept than anything else. For now though, the XMB's functionalities encompass Recovery, Settings, Photo, Music, Video, Game, and Network.