Depending on your users and network configuration, a web server may be required. Qlikview has its own implemented, but some people has already runing IIS and QlikView has some scripts to make it work with that instead its own server. The AJAX client for example will always need a webserver. The plugin may or may not need it.
The IE Plugin is one of the several clients QlikView has to get to the server and render the dashboards in the documents. It's not the only one, AJAX has been improved significantly in version 10, there's a touch client for tables (not only the iPad), there's a java client for Blackberry, an iOS one for the iPhone and iPad... and the Desktop application itself has the "Open in Server" option in the Start page.
In short, the answer to your question is yes, you can use the QlikView Web Server (you will be prompted about it during the server installation process) and you can make your users use the AJAX client, that requires no installation (other than the browser, of course) on the user's side.