I'd like to add my favourites to the list, but as the ones I use on a daily basis were all written by me, I'm afraid I might come across a bit biased...
Uploadpdf assignment type - I collect all my students assignments that way (they submit them as pdfs) and then mark them online, giving them feedback directly on their work.
Uploadpdf grade report - that helps me to keep track of work submitted, late/overdue work and the marks gained for each submission (as well as a list of recently marked assignments, to update our central college markbook).
Checklist - to help students to keep track of what they need to complete for each assignment and so I can see how far through each of them are (assuming I can get them to keep their checklists up to date).
Aside from that, most of the other bits I use regularly are part of the standard moodle install.