Hearing about Command-Shift-D for sending emails reminded me of a tough lesson I learned in Gmail. When you turn on the extended keyboard shortcuts in Gmail’s web client, Command-Shift-D deletes the draft you’re working on. I couldn’t figure where all of my responses were going for a long time, and I thought I was actually sending emails.
When @MacSparky brought up component-based applications, I was a bit surprised that @ismh didn’t mention OpenDoc. 
BTW: for anyone who is looking for terminal tips in podcast form, there’s the Taming the Terminal podcast by Bart Busschots and Allison Sheridan.
My favorite Mac keyboard shortcuts are to use ^1, ^2, ^3, etc. to switch between virtual desktops in Spaces. I’m not sure how well known it is that you can do this because I keep hearing people complain that they don’t like using Spaces because it’s such a pain to “swipe between desktops”. Good news — you don’t have to!
It’s a bit hard to find the setting in System Settings so I’ve attached a screenshot:
Quick note to @ismh regarding paper cuts, you can use archival cotton gloves so your hands can breathe. I use them to work with print proofs, photos and negatives. They work great and my hands don’t get sweaty or leave fingerprints. And, you can throw them in the wash if you’re not worried about lint specs. (I don’t for the ones I use with negatives.) Tip: get good quality ones that are form fitting, fit snugly and have elastic at the wrist. Just my 0.00000114BTC