I only use 2 widgets:
1. Weather Bug for temps and radar at a glance so I know what's going on without loading a website
2. Two instances of the gmail inbox app. That way my wife and I can see our inboxes without logging out/in each others' accounts manually. Basically, if you hit F12 and then click on a new email, it will open it up in Safari. That's obvious and not a big deal, but what happens when you click on an email from another account in a separate widget instance? Yep, it effectively logs out the other user and logs you in under the new user. VERY useful and time-saving, because we don't need two separate OS X accounts clogging up our laptop hard drive just to be able to easily check two separate email accounts.
I use Dashboard all the time, but only for those two things.
I also have iStat running, and I guess I look at it every now and then, but not because I need to. I just like numbers and graphs and charts and stuff. And wires. I really like different sorts of wires and cables.