I wrote malloc.fth for Mitch Bradley's ForthMacs, which ended up in OpenFirmware:

https://github.com/openbios/openfirmware/blob/d5cc657ce81c0f...

    \ Forth dynamic storage managment.
    \
    \ By Don Hopkins, University of Maryland
    \ Modified by Mitch Bradley, Bradley Forthware
    \ Public Domain
    \
    \ First fit storage allocation of blocks of varying size.
    \ Blocks are prefixed with a usage flag and a length count.
    \ Free blocks are collapsed downwards during free-memory and while
    \ searching during allocate-memory.  Based on the algorithm described
    \ in Knuth's _An_Introduction_To_Data_Structures_With_Applications_,
    \ sections 5-6.2 and 5-6.3, pp. 501-511.

Forth systems running on C libraries can also call back to the native malloc/free implementation, but it's nice to have a pure FORTH implementation for embedded applications.