What are the various ways to access devices on client PC - a barcode reader, a scanner, etc. - from a browser? I realize my users may need a plugin. These devices may have an API that is specific to their device; I would like to exploit their API if available (maybe Java, maybe C, maybe command line).
A signed Java applet can have platform-independent access to most things. The user has to have Java installed, and has to allow the signed applet to run.
If you write a plug-in to the NSAPI, once they've installed it you'll have access to essentially anything the user can access. Of course, they have to install the plug-in.
Flash offers quite a lot of access to devices.