Flickr
flickr.com
This public showroom for personal pics just might be the fastest-growing social network on the Web, and it's completely addictive. You upload your images and assign each an identifying tag; these tags help other members find your stuff, and you theirs. You can join groups and create new ones, post comments about particular images and designate favorites. Free membership is limited to 20 megabytes worth of uploads per month. Turn Pro and pay $25 a year for a host of other perks.