Update: The overall feedback seemed positive, so I started the thread here.
I propose having a meta thread with the title "What third-party packages do you use?" (see proposed question in the quote). Would this constitute and abuse of meta? What do you think about the appropriateness of such a question on meta?
Sometimes I come across some very useful packages that people mention in their answers. I wish there were a central list where we can post the most-used package and let those who use them vote. The rule would be: only post packages you made yourself or packages you use yourself. Upvote those that you use.
The best and most commonly used packages would float to the top. This list would make it much easier to discover useful, good quality and up to date packages. Digging these up from the Wolfram Library Archive (previsouly MathSource) is not that easy. MathSource is a huge repository of good and bad, new and outdated, big and small packages. It doesn't give the reader and easy way to find which packages are still maintained and good quality. It is only useful if I am specifically looking for something (e.g. MATLAB integration) and I am willing to try out / try to get working any package on the topic. There are no reviews and no guides on selecting.
This meta thread would be a crowd-source recommendation list which does not aim to be comprehensive, but should make it much easier to discover packages that can be of interest. The purpose of the thread would be not to have a comprehensive list, but to highlight good packages that several of our members use in practice.
To mention a few packages I learned about through StackExchange: LevelScheme, CustomTicks, NCAlgebra, etc. There are a number of other ones, e.g. a package for creating "Applications" without WorkBench, that I remember reading about but can't find now.
I believe such a list wouldn't grow too large to be unwieldy.
Let's use this thread to post our favourite third-party packages.
Please post only those packages that you actually use yourself (not any package you found) or packages that you created yourself. Each answer should be one package. Upvote those packages that you use yourself.