Excursus for anyone interested in the development of open content licenses
When the spirit of the open source movement finally influenced the world of content, the first corresponding licenses were developed. The first one, called Open Content License (OCL), was created in 1998 but was replaced by the Open Publication License (OPL) one year later.
The first creators of these licenses were learning experts, not legal experts. So, the two licenses following in 2002, GFDL (GNU Free Documentation License) and CC (Creative Commons), built on the innovation of their predecessor. In the best sense of “OpenContent is dead. Long live OpenContent”, the creator of OPL acknowledged the quality of Creative Commons in 2002 declaring that no future development would be carried out on the OPL license. (source)
GNU is, at its base, an open source license which has been developed further for licenses for other creative work. GFDL and CC-BY-SA are almost identical except for technical differences. The main difference, as Creative Commons states, is that any copy of a work licensed under the GFDL must include a full copy of the license. That's not a big burden in software, but if someone is printing off a GFDL document, they are legally obliged to include a license printout which may be as long as the document itself. The CC-BY-SA is much simpler, requiring attribution as requested by the source, which typically is the source's name (e.g. "Practical Action," "Appropedia" or "Jodie Smith"), perhaps a title, and a link.
Further reading: "GFDL vs. CC-by-sa" (click!)
One more interesting development concerning open licenses in Europe is the UK’s Open Government License. It applies to many but not all so called ‘Crown Copyright works’ and is compatible with the Creative Commons Attribution (CC-BY) license. The license may be applied to texts, media, databases and source code. Organisations at various levels within the UK government use this license and those reusing their work have to acknowledge its source.
Further reading: "Open Government Licence" (click!)