The site was originally created with a focus on TikZ, but Andrew always intended it to be an open repository for community-developing of packages from TeX.SX answers.
See these Meta threads for explanations and references:
It seems I'm now the pioneer for adding a non-TikZ package, and to feel less lonely, I'd like to invite other contributors to join with packages developed from TeX.SX answers.
This is not intended for packages which can immediately go on CTAN, but for code which is either more of a proof-of-concept or needs polishing before being presented to the anonymous user.
I'd like to stress again the basic idea that packages on the Launchpad site are presented to the community of (approved) developers for discussion and improvement, so it's not just a container to dump "your" stuff, but a medium for mutual development.
This question should be community wiki and should be improved over time.
As for "What are your favourite TikZ/PGF answers?", I suggest to add potential candidate answers which could be (or are) made into packages for the launchpad site as answers to this question.
Please also visit the chat room dedicated to the site.