Transformational geometry software I'm doing a third year Geometry course involving transformations and isometries of the Euclidean plane. We're learning about the four isometries - rotation, reflection, translation and glide-reflection.
Is there any (free, preferably open source) software out there that can help me visualize these sorts of problems?

The free program Geogebra does all of these things. Versions of it are available for a wide variety of platforms. I have used it quite a lot, mainly for its graphing capabilities.
People have produced large numbers of worksheets that will enable you to explore transformations, and many other things geometrical. 
