Apache is still top of the web server charts with just under half of the top 100 US websites running on the open source software.

Researchers looked at the top 100 US websites, according to Alexa.com.

By looking at the HTTP response headers they were able to identify the operating system for all but 11 of the websites.

Apache was behind 49 of those sites with Microsoft's IIS in a strong second place with 20.

Lighttpd accounted for four of the top 100 websites. LiteSpeed, nginx and Sun's ONE Web Server had two each. Ten websites were running other kinds of software and 11 were unidentified.

The survey, from Pingdom.com, warns that some websites put incorrect information in headers as a security measure.

The figures broadly reflect monthly returns from Netcraft - in February it received responses from 158 million sites, an increase of 2.6 million sites since January.

Netcraft has Apache providing 51 per cent of the software running these servers. Figures for Apache are on the increase again after falling since late 2005. Netcraft also saw growth for LiteSpeed - up 10 per cent in a month.

But the largest growth was for AOLserver - up from 35,000 in January to 105,000 in February, despite not being updated since 2006. The majority of the new sites using AOLserver were in Poland. ®

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