Activity shows what the people you are following are doing on Twitter, from following to RTing to favoriting to adding people to lists. (It does now show unfollowing, which is rude. Don't unfollow anybody.)

Stellar is a website built by Jason Kottke. You associate your Twitter, YouTube, Vimeo and Flickr accounts, and anytime you star something, it shows up in your favestream. My stream looks like this: You can follow people on Stellar to see their Twitter, YouTube and Vimeo faves.

Since it's a little hard to remember to visit a website daily, and the RSS feed can be overwhelming, someone built a bot in PHP that RTs the tweets the people you follow on Stellar fave. (Got that? You follow people on Stellar. Then you see what they fave on Twitter. The tweets they fave, thanks to that bot, get RTed into your stream using a separate, private account.)

I rewrote Mike Davidson's bot from PHP to Node.js:

These Twitter accounts known as Stellarbots, and should be set to private, otherwise it will automatically RT stuff, and auto-RT are annoying.