This is a compilation of groff examples shared on the mailing list or detailed in the different man pages. This is aimed to be a crude help if you want a particular hack.
Of course, man pages and info pages are the main source of information for general help. The following pages have been of tremendous help to me: groff(1), groff(7), groff_char(7) and groff_tmac(5).
Linked below, you can find the project page of groff, of mandoc and of mdoc.
If you think a macro should be in this list, if you found a mistake or care to give some feedback, send me an email: thomas AT dupond DOT eu.