I love using maps in my research, and although the National Library of Scotland is usually my first port of call for most Scottish maps, there are also some fantastic finds from other archives.
One of my favourite sets of maps is the 1769 estate plans of the Breadalbane estate on Loch Tay side. These are superbly detailed and have useful accompanying information in the form of a survey book published by the Scottish History Society ["Survey of Lochtayside 1769" (Scottish History Society, 3rd series, vol. 27)].
Even if your ancestors did not hail from this area, the maps are lovely things to look at in and of themselves, and they are freely available to view online. Click the links to view images of The North Side of the loch and The South Side of the loch at the Scotland's Places website.