I recently read an article about how "Mail Goggles" have been inserted into GMail as an option from the GoogleLabs. What are Mail Goggles? Well, it is a program that forces you (if you choose to activate it) to answer a set of relatively simple mathematics questions before sending out an e-mail during the night or on weekends. (The assumption is that if you are inebriated or otherwise without mental filters, you might not be able to answer the posed questions, and thus don't really want to send out that screed you just penned.) I don't know what happens to the mail that doesn't get sent, but it I'll test it later (either tonight or this weekend) to see what happens.
In addition to the "Mail Goggles" thing, there are several other ones that I added to my own private [Idaho] Gmail, including "Quick Links" (which provides obvious URLs to links in your mail), "Pictures in Chat" (which should be self-explanatory), "Muzzle" (which eliminates people's chat statuses), "Right-side Chat" (which puts the Chat box on the right-hand side of the screen), and "Right-side Labels" (same as Right-side Chat, but with labels). Now all I have to do is get used to how my screen looks when I startup GMail.
In addition to all these great things from the Google Lab, I also added the Firefox add-on "Better GMail" which (among other things) provides file icons for attachments and different skins for GMail.