Currently we only have "When an item is created or modified" please add a standalone "when an item is modified"
Sometimes you just need that bit of standalone.
I was also looking for this and found this link where a good workaround is discussed:
Use the created or modified trigger followed immediately by a condition with this logic (edit condition in advanced mode)
That does work, but not on lists where I had other workflows that run and make changes when items are created, since those workflows would then trigger this workflow anyway.
I will still say that an actual "when existing item is modified by" trigger would be nice if only to slightly reduce workflow calls. Even more useful would be a workflow that could be triggered only when specific fields are changed.
Edit: See this idea