The Bothy network is an amazing and free way to explore the remote corners of Scotland. Bothy's are normally croft houses or cottages that you can stay in for free and work similarly to the hut network in New Zealand. People often replenish supplies such as tea, coffee and firewood and they rely on peoples honesty and courtesy to work. Many of these buildings have barely changed in the last century and its still possible to cook on a Range in some of these buildings.
To find a bothy, the Mountain Bothies Association website is probably the best place to start and is also a great place to read a bit about how they came to be, and the philosophy behind them. With the bothy networking being free, this is also a great alternative for those who enjoy freedom camping and it helps open up the outdoors to those who might not have explored Scotland otherwise.
Although they are not bothies, the Landmark Trust has a wide range of croft houses and remote cottages. The Trust puts a lot of effort into making the properties as historically accurate to how they would have been in years gone by.