In the beginning there was DIGG. Which begat the DiggArmy, which sucketh both mightily and hard.
Actually, DIGG was far from first (Slashdot was far earlier and has a similar 18-30 year old males living with parents vibe; Fark is in the same boat, too) but it provided my first run-in with a large group of people (largely men, largely aged 18-30) who have nothing better to do than maliciously crash websites, start flame wars, and bait newbies -- not necessarily in that order.
DIGG bugs other people too: my hubby, Ryan Tomayko, and Wil Wheaton, among others.
Recently, I came across KiRTSY, the grrl-positive alternative to DIGG. You may have noticed the little "ADD to KiRTSY" button below each post. I really like this site because categories that make sense to me, including arts & crafts, and it seems... well for the moment anyway, a happy place. We'll see how long that lasts.
If "green" is your thing, there's HUGG -- which actually bills itself as "DIGG for green" -- personally, I'll pass; I get my eco-news from Green Daily.
If you're really curious, check out this long list of DIGG clones -- though be warned, many have already failed, bringing up 404 pages or domain parking lots.