RMD

RMD is a variation of RenderWare's node-based model format. It comprises all of the 3D assets in the PS2 versions of Persona 3 and Persona 4. Currently, it's possible to edit textures, animation ordering and meshes, but rigging custom RMD models is not yet possible.

RMDEditor
RMDEditor.exe is a program created by OfTheSeventh and maintained by ThatTrueStruggle. It features a GUI similar to BrawlBox and is capable of opening PAK/BIN, RMD. ACX and SPR files, as well as exporting and importing nodes from the files. You can download the latest version (v1.4.6) here.

Amicitia
Featured article: Amicitia (program) Amicitia is TGE's all-in-one editor for Persona formats. Currently, it supports almost everything that RMDEditor can, but without having to deal with nodes. It also features texture conversion from PNG (for both TMX and RMD). You can get it here.

Trivia

 * Other games including Sonic Heroes, Shadow the Hedgehog and Grand Theft Auto III customized versions of the same format as well. A list of games that use Criterion Software's RenderWare model format can be found here.
 * Grand Theft Auto III uses the same Animation format, referred to as "hanim animations."