The software on this site is licensed under a variety of OSI-approved open source licences. UnityWiki is unambiguously a derivative work of PikiPiki, which is covered by the GPL, and hence is distributed under the same licence. The two original projects, Bookaroo and Newfile, are licensed under the MIT Licence (basically equivalent to the advertising-clause-free version of the BSD licence).
Ideally, I'd like to distribute things under a slightly more restrictive licence; one that forced derivative works to be licensed under similar terms, but didn't impose any restrictions on other work that is merely combined with it in the way that the GPL attempts to (I think that the GPL is on shaky ground there, as it seems to try and extend the definition of "derivative work", but in any case I'd rather have a licence that explicitly permitted distribution of combined works under different terms).
The LGPL is sort of right, but I don't like the wording; it seems overly complicated, and overly specific in places (for example, it uses the words "object files" - not a useful concept if you're trying to licence software written in Python, or even Java). Apparently, the Mozilla Public License 1.1 would do what I want, but according to the Free Software Foundation it's not compatible with the GPL, and the resulting restriction is as bad as a restriction to only link with GPL-compatible software. The Mozilla project themselves get round this with a triple-licensing approach (MPL/GPL/LGPL), but that seems a bit messy. Hence, as BSD-style licence seems to be the best bet for now - it also has the advantage that it doesn't limit your options in the future. Maybe this is the way to go long-term; after all, I'm not trying to push any free-software agenda or anything. It'd just be nice to ensure that improvements to the software could be incorporated back in to the main tree.
If you've got any suggestions or comments about this issue, I'd be glad to hear them.