None of these are my packages and it may be best to let the authors contribute there own but here are a couple of related packages, not all necessarily extensions, that I'm aware of...
- ggtern
- GGally
- gganimate
- ggthemes
- sjPlot also uses ggplot2 for such things as diagnostic plots of regression models, but may not be considered a true extension as its more a wrapper.
None of these are my packages and it may be best to let the authors contribute there own but here are a couple of related packages, not all necessarily extensions, that I'm aware of...