I've been using StumbleUpon for more than a year now and with that span of time, I can say that this Internet tool is really amazing and helpful in discovering whatever interesting in the Internet today. The fact that when you install the toolbar and just click the "stumble" button and then direct you to a page that is exciting and creative or awesome is such a brilliant idea. In addition to that, StumbleUpon really is so addictive that I often sleep late at night (early morning like 2AM actually) and just continue stumbling. Every web page and site I discover is just brilliant.
You can try it yourself if you want. Just go to StumbleUpon website and install their toolbar. You can also search them through Firefox add-ons.