iHTML is a commercial product developed by Inline Internet Systems of Canada. As Inline's main business was the development of dynamic web sites for corporate clients, it appears iHTML was born as an in-house (homebrew) product. As of today, its focus has shifted to provide e-commerce software and development tools for small to medium-sized companies.
iHTML supports all major webservers: Apache, IIS, Netscape, Zeus and others, and this on several platforms. It does not boast innovative new features (such as full object orientation), but does support the basics that everybody needs such as sessions and persistent database connections. However there are some new twists such as Back Page Processing, which allows for running parts of a site without actually displaying them to the visitor.
iHTML has been used by high-profile companies such as Xerox, Raytheon and Intel, but is also available in a free version.
The main advantages of iHTML are the nice support of a dedicated company, the easy integration with databases, email servers and dynamically generated graphics. Furthermore, the concept of the language itself is said to be kept close to HTML (something of which I am very sceptical).
My personal take: there is not much that iHTML can do that it's competitors can't. However, its always nice to have an integrated package (i.e. no need to install separate modules) that is multi-platform and well supported. I feel that Inline missed the ball by not supporting an object oriented programming approach, which is the direction taken by JSP and the newer versions of PHP, among others.