I think a platform independent solution would not be easy.. unless going for java.
But maybe doing 2 platform dependent ad-ons (1 for NS and 1 IE) is a simple thing, with great value added. We have browser detection so for the user it would be transparent.
I never did any thing with activex controls, that I belive mail.yahoo.com is using them (thanks David). Does anyone have any experience in them?
Is there code available that we can plug to OpenACS?
