So I found a bunch of 2D isometric engines, but I don't know if they're any good.
They are: Wade, Isogenic, Wurfel, Fife, Ezelia, Construct 2 and the list goes on.
Similarly free 3D engines seem a little harder to find in quantity than 2D engines.
http://www.worldofleveldesign.com/categories/level_design_tutorials/recommended-game-engines.php