"Open-source programmer Harald Welte spends a lot of time
enforcing provisions of the General Public License, taking legal
action when companies include GPL software in products without
releasing their versions of the underlying source code. He's had
many successes--most recently in court against networking equipment
maker D-Link in September--pressuring several companies to comply
with the GPL's requirement to provide their source code.
"The idea of the license is that those who benefit from the
freedoms of GPL software--it may be copied, modified and
distributed--must also provide their source code changes for the
benefit of others using the code..."