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Apparently, you can use flash by installing firefox-bin-flash and nspluginwrapper. I've not been able to get it to work (I think there are some issues with Flash v9), though I've not tried too hard because I don't really need it that badly. It _is_ possible to run it, though.
Quickly searching, I found this: http://mail-index.netbsd.org/pkgsrc-users/2007/02/05/0010.html
Gives a nice step-by-step to get flash working. I'll have to try it...
As for Java, I think it does work on NetBSD.