If you have an Apple Developer Connection membership (a free membership is available), you can download a free trial copy of WebObjects 5 for Mac OS X.

Go get it! (download the one with the 10.0.1 developer tools)

Unfortunately there isn't a trial version available for Windows. I'm assuming that's because the developer tools on Windows are based on YellowBox which uses the Adobe Display Postscript Windowserver which Apple has to pay Adobe a royalty for. Bummer.