Quote:

NetBeans (#1 platform independent IDE framework)
Apache WebServer (66% market share, 53% of secure installations).
Sendmail (#1, 42% share over Exchange 18%)
BIND (over 95% of reverse DNS lookups)
PHP (#1 server-side scripting language)



But of course you can't describe any of those as being mass market in the same way as a hardware MP3 player. In the grand scheme of things very few people ever buy a webserver, email server, name server or programming language. These could just as easily be considered niche market products.
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Remind me to change my signature to something more interesting someday