Netcraft tracks the trends of JSP usage:
Last December we reported that the number of JSP sites had grown quickly during 2002, albeit from a small starting point. Reviewing the growth over the past year since July 2002, JSP continues to enjoy fast growth with a 94% increase in ip addresses running JSP based sites to over 44,000 ip addresses running some 105,000 active sites.
Check out the rest of the article. The makeup of operating systems running JSP is quite interesting.