What I use for development of HTML, JAVA, PERL, C, C++. Very stable, but does not have much software that can run on it, because of lack of knowledge about it on the part of the general public. Great for webservers, but until people decide to create software for this system, buggy, backdoored systems like windows will have to do. Linux is often used by professional developers, webmasters, advanced computer users, and people who are able to get off their arse and install it for the sake of their privacy. Some versions (i.e. Red Hat) are relatively easy to install, and can be found free in libraries. Other 'distros' can be found on the internet, but may be harder/easier to install. It can be installed on very low end computers with under a GiG of drive space, and 32k ram. So stable, a Win 9.x. demo cannot crash it =)). Not used often because many people have no idea it exists (that's changing though).
a good OS for stability.