I love maze algorithms. The visualisations by https://bl.ocks.org/mbostock/11357811 are great for understanding the basic algorithm, and I like the twists in the article.
Never sure how much promotional content should be on here, but the company I work for (Redgate) has a coding challenge related to this. See http://www.red-gate.com/our-company/entrypage/coding-challen... if you are interested.