Aria is good but not a replacement for correct HTML. This Aria has some examples of the good and the bad.
WCAG has techniques PDF1 to PDF23. Here are some examples on the PDF test page..
Various examples in the many ways you can create and break links.
More annoying than links when broken, here are some email tests.
MS Office is used everywhere and we have an MS Office test page containing a variety of issues.
Specific tests for mobile devices including issues, such as, orientation.
Embedded video test page showing the difference between just embedding a load of YouTube videos on a page (which results in duplicate ids) and using a helper such as lite-yt-embed.
Brand brand test page containing variations of names to be used with brand testing.
Performance performance test page containing large files for performance testing.