I use StumbleUpon just as far as that I have a button on my toolbar & I stumble posts that I like. I don't have a button on my page, but it's one of the things I'm thinking about.
I also use Delicious a little bit, but not a lot.
Those are 2 things that I keep seeing everywhere - along with Digg - but I don't feel like I have the time or energy to put much effort into those right now.