Large and Mainstream Networks:

• MySpace
• Bebo
• Multiply
• Vox
• Xuqa

Widgets and add-ons

• Slide.com
• RockYou
• ZingFu
• Stickam
• MyBlogLog
• Zwinky

Social News and Social Bookmarking

• Del.icio.us
• TrailFire
• Digg
• StumbleUpon
• BlinkList

Photosharing

• Flickr
• Zooomr
• ImageShack
• Tabblo
• BubbleShare

Videosharing:

• YouTube
• Gotuit
• MetaCafe
• MotionBox
• Revver

Startpages

• NetVibes
• PageFlakes
• YourMinis
• ProtoPage
• WebWag

And my personal favorite at this moment, duh… just added this nice feature to my site, Dapper. Using Dapper, any web page becomes a LEGO block, which you can arrange and manipulate in any way you like.