Maybe you have a blog, a Twitter page, a Flickr photostream, and a bunch of other sites. And maybe when you post to one of your sites, you want the post to appear on all your other sites automatically.
I currently do this with RSS and Atom feeds and some Yahoo Pipes magic.
Flowchart of connected sites
If the web site has an RSS feed (and which site doesn’t?), it is very easy to connect it to Twitter, identi.ca, and other social networks by using Twitterfeed. Basically, you create an account on Twitterfeed, tell it the address of the RSS feed, connect it to your Twitter account (using OAuth), and whenever there is a new post on the RSS feed, it will be posted to Twitter.
My Google Profile also has an RSS feed, and it already pulls in content from a variety of sites, including my blog, my Flickr photostream, my Google Reader Shared Items, etc.
In order to connect a site to your Google Profile, you first add a link to it in the profile settings. Make sure to tick the box that says “This is a profile page about me”. Then, on the linked site, you add an XFN link with a rel="me" attribute back to the Google Profile. This can be either a visible link using the <a> tag (which is what Twitter and other sites use for the web link that you specify for your profile page there), or using the <link> tag inside the page header:
Älgspotting Blog
Flickr Photostream
Google+
Spotify
YouTube