Update 2015 Sept: See also PackageData.net, and consider adding there as well.
Let's use this thread to post our favourite freely available 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, and should preferably contain a short description/review of the package.
Upvote those packages that you use, so the best ones will float to the top.
The purpose of this thread is not to create a comprehensive list (such a list already exists) but to highlight the best / most popular packages that many of us could find useful/interesting. We can also use this thread to share packages our members have created. Please see discussion here.