From the webpage:
Wikipedia offers free copies of all available content to interested users. These databases can be used for mirroring, personal use, informal backups, offline use or database queries (such as for Wikipedia:Maintenance). All text content is multi-licensed under the Creative Commons Attribution-ShareAlike 3.0 License (CC-BY-SA) and the GNU Free Documentation License (GFDL). Images and other files are available under different terms, as detailed on their description pages. For our advice about complying with these licenses, see Wikipedia:Copyrights.
I know you are already aware of this as a data source but every time I want to confirm something about it, I have a devil of a time finding it at Wikipedia.
If I remember that I wrote about it here, perhaps it will be easier to find. 😉
What I need to do is get one of those multi-terabyte network appliances for Christmas. Then copy large data sets that I don’t need updated as often as I need to consult their structures. (Like the next one I am about to mention.)