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.

For Ada, I like WikiBooks: http://en.wikibooks.org/wiki/Ada

Also useful
http://www.adaic.org/
http://www.sigada.org/
http://www.adacore.com/
http://www.sparkada.com/
https://libre.adacore.com/ (free GPL'd Ada 2005 compiler)