Above are the survey results produced by Netcraft (http://www.netcraft.com/survey/) of the top servers up to March 2001.
Apache servers have over 60% of the market share, and have done for some time; plus the trend is still heading generally up. Microsoft's IIS is gradually declining after a fairly recent surge, presumably due to the huge number of high-profile web sites that have been defaced using security holes in IIS in the past few months.
If you're not aware of what server your ISP or Web space provider is running, you can use the following form (when you're online) to get Netcraft to check the web server and operating system of either the main domain (e.g. www.argonet.co.uk) or any domains you own (e.g. www.goodwin.uk.com)