Plugin: editdiff
Author: JeremieKoenig
Included in ikiwiki: yes
Enabled by default: no
Included in goodstuff: no
Currently enabled: no

This plugin adds a "Diff" button when a page is being edited. When clicked, a diff between the stored page and provided content is shown in the "Page Preview" area.

Problems

No special handling is done of concurrent edits: changes introduced independently will show up in the requested diff, although they will be merged when the page is saved. I suspect even detecting this case would require changes in the RCS backends.