AS400 Emulator or Virtual Machine

No. This is highly proprietary and I do not know of one.

You might find http://www.holgerscherer.de/cgi-bin/db2www/rz.nd/rz?menu=23&language=ENG interesting.

The best you can do is get a cheap, used AS400 on ebay or an LPAR on a larger machine.

Emulating an AS/400 is not possible, but if you just want to poke around,

http://pub1.de/ / http://pub400.com/ offers free playground-level access to AS400s for personal use.

I just made an account and am having a (very fascinated!) look around. I have no idea what I'm doing (and no technical support is offered by the service) but it's very fun, and I can't break anything!

The service is offered by http://www.rzkh.de/.

_{NB: SSL is an advertised option (over port 990), but tn5250j doesn't seem to connect properly (it connects, then hangs). Only port 23 works. Perhaps a temporary glitch.}

There used to be a PC based emulator named Baby/400 but I can find no recent information about it.

Try the mailing lists at midrange.com.