In the .NET world ActiveX is for the most part dead (even though we can talk to it, we try not to).
Instead one possible option (that only works in IE) is to host a .NET user control on a web page and use it through IE.
Google accomplishes this without ActiveX, Flash, or Java.
It done by setting up a converstaion via XML between a Javascript routine on the browser, and the webserver. The technique is known as "AJAX" (Asyncronous Javascript And Xml).
Many open-source .Net libraries to handle Ajax are becoming available now. Microsoft's own is called "Atlas" (http://atlas.asp.net)