JWepGen is a WEP key generator written in Java.
Currently it generates 128 bit keys correctly and I think it's doing the 64 bit keys correctly, I can only verify the first of the four keys.
It is based in part upon the XWepGen program for OSX and in part on the nwepgen utility that's part of the Linux WLAN project.
This software is distributed under the terms of Apache License version 2.0. This code includes software that is part of the Jakarta Commons project (namely the hex encoder).
You'll need Java 2 version 1.4 or Java 5 version 1.5 available to run this software. With that installed, download the newest jar file below and save it someplace useful (like My Documents or your home directory). To run it just double-click it in windows. On other platforms or from the command line just run java -jar jwepgen.jar.