There are lots of them at lots of scales viewable online - free of charge - at the maps section of the National Library of Scotland website but I would have to say the NLS website scores only 6 out of 10 in terms of navigability in my book so the purpose of this post is to showcase what's on offer and give you the direct links:-
OS Quarter Inch (1921-23)
Bartholomew Half Inch (1926-35)
From that link, you can link to the 2nd (1885-1900) and 3rd (1903-12) editions of the one inch map. These three editions are all black and white, thus:-
With the above maps, when you click the sheet you want, it opens in a new window. I recommend the "View with Plug-in" option (he says like he knows what a plug-in option is - just press "yes", "accept" etc.) which enables you to zoom right in and drag the map around the screen etc.
For the OS One Inch "Popular" edition of the 1930s (coloured), you just keep zooming in on the same window with no plug-in.
For the next scales up, 6 inch and 25 inch, you can just keep zooming in but a preferable option is, once you've got to a scale which shows the boundaries of the sheet, click "By point" (top right) and then click your desired location on the map. This will highlight the sheet in purple and top right of the map you'll see a link like Ordnance Survey Map - Argyllshire - 1:10,560 Sheet CLVI. Click that to take you to the map.
OS 6 inch (1st ed. 1843-82)
OS 25 inch (1st ed. 1855-82) - there isn't total coverage of Scotland at 25 inch scale yet. I assume it's a work in progress and the remainder have yet to be scanned and uploaded. Meanwhile, there are 13,045 sheets to be perused. Including Scalasaig.
There are also Admiralty Charts
And this is what Scalasaig on Colonsay looks like from the air courtesy of Bing Maps
Nice wee place, Scalasaig, I commend it to you all.