Netcraft's survey of SSL sites has now been running for over ten years. The first survey, in November 1996, found just 3,283 sites; since then, the number of SSL sites has had an average compound growth of 65% per annum.
The survey is a good guide to the growth of online trading and services. The survey counts sites by collecting SSL certificates; each distinct, valid SSL certificate is counted in the results. Each SSL certificate typically represents one company's details, and each certificate must be approved by a certificate authority, so the data is typically more consistent and less volatile than other attributes of the Internet's infrastructure.
Netcraft: Internet Passes 600,000 SSL Sites.
This is many fewer than the total number of websites, current Netcraft web server survey, as I access it now in May 2007 it reads 118,023,363 sites.