The National Library of Scotland has been posting old OS maps online for a while now, and the coverage is improving all the time.
With some maps, including the 1st Edition 6" maps, you can switch between the modern Google view and the old map, which is handy for identifying features you have seen in real life or for seeing what remains of a feature from the old map.
There is more or less full coverage of the 6" to the mile maps, with two or more revisions usually available for the 2nd Edition. Coverage for the 25" to the mile maps is improving, and again there are usually two revisions available.
Also worth mentioning is RCAHMS's Canmore Mapping service.
It's a bit fiddly to use at first, but it allows you to zoom in on an area and view any online RCAHMS content for your selected area.