The ubiquitous sidebar.
It is a staple of the blogosphere. Assuming that you are reading this post on WPMU (and not in an RSS reader), the tiniest of glances to your right will reveal our sidebar, complete with various widgets and graphics.
Sidebars are perhaps so popular for their flexibility. You can put just about anything in a sidebar — the world is your oyster. The problem is that far too many blogs abuse this freedom too willingly and end up with a mile-long monstrosity nestled alongside their content.