From Humanist I just learned about Juxta Commons. This is a web version of the earlier downloadable Java tool. The new version still has the lovely interface that shows the differences between variants. The commons however, builds on the personal computer tool by being a place where collations can be kept. Others can find and explore your collations. You can search the commons and find collation projects.
Another interesting feature is that they have Google ads if you search the commons. The search is “powered by Google” so perhaps that comes with the service.