Noone has mentioned it yet, but grail is/was pretty cool
Grail was a free extensible multi-platform web browser written in the Python programming language. The project was started in August 1995, with its first public release in November of that year. [1] The .3 beta contained over 27,000 lines of Python.[1] Its last release was of version 0.6 in 1999, with latest version under development.
