ReplicatorG is a machine-controller developed by Makerbot, which integrates Enrique's skeinforge.
Initial efforts to make ReplicatorG work with the RepRap 5D versions of the firmware (vanilla RepRap and Ultimaker) were undertaken by ErikDeBruijn and D1pl01d. There's now a version that includes checksums and error recovery (D1pl01d's work) and generally has many improvements such as supporting different build volumes, etc.
ReplicatorG was abandoned shortly before Makerbot was bought by Stratasys. A fork is continuing development.