The Scottish Medievalists’ flagship project, the Atlas of Scottish History, is now being hosted by the Society of Antiquaries of Scotland (more details to follow).
The fruits of another long-running project, the Syllabus of Scottish Cartularies, is also freely available here.
Another Society project, A handlist of Scottish sheriffs to c. 1306, is now available here.
A complete Calendar of Scottish Charters and other Acts from 1094 to 1286, produced by the People of Medieval Scotland project team, is coming soon.
The Scottish Place-Name Society (SPNS) maintains a very thorough and useful page linking to many resources of interest to medievalists here.