MikuMikuMoving GUI
MikuMikuMoving, often abbreviated to MMM, is an alternative program to MikuMikuDance and is developed by MoggProject. It can be downloaded here.
MikuMikuMoving is seen as a more beginner-friendly alternative to MikuMikuDance. The program features a streamlined UI that allows users to easily locate and experiment with different settings for their project.
Because MikuMikuMoving is a newer program than MMD, it features options that aren't available in MMD, such as compatibility with .MP3 files and support for soft-body physics.
While MikuMikuMoving is a newer program, its functionality is limited compared to MMD. MikuMikuMoving is not compatible with MMD's project format, .PMM, and instead uses its own file format, .MPJ. This means you couldn't save a project in MMD and try to open it in MikuMikuMoving, and vice versa. MikuMikuMoving is, however, compatible with .PMD/.PMX files, as well as .VMD/.VPD files, so you can use the same models, motions, and poses between both programs. MikuMikuMoving is incompatible with a majority of effects made for MMD, though there are a handful of effects that have been made available to use in MikuMikuMoving.
Documentation and tutorials about MikuMikuMoving can be harder to come across as it has a much smaller following than MMD.