Actually, GCC's Ada compiler (GNAT) is written almost entirely in Ada. I think it was originally bootstraped in the early 1990s using a different Ada compiler.
I use GPS available in the GNAT GPL packages at http://libre.adacore.com/libre/. It supports Ada, C, C++, Python, SPARK, etc. Its probably the best free IDE for Ada and SPARK, especially for use on large projects.