To complement Luca’s answer, I’d say ActiveX is supported only by Internet Explorer and Applets are supported only by IE and Firefox. Anyway there are (not so) fresh news about Applets: Oracle are deprecating Applets on Java 9. Have a look at Moving to a Plugin-Free Web.
There are solutions that use a scanner service to capture scanner images. Since Java 7 there is Java™ Web Start. This is kind’a Microsoft ClickOnce. The problem with ClickOnce is that it only runs on Windows. But this didn’t stop Google from using ClickOnce for the Chrome installer.