Question: I see a lot of blogs that seem to have really cluttered sidebars, with a ton of additional widgets, navigation, etc, that I never find myself using. What are some tips for things that make good additions to the sidebar, and some things that are unnecessary? 

One example that springs to mind are Tag Clouds. I see these everywhere, but how often to people actually use them?

Answer: 
I never use the tags cloud. Vote up if you do not use them as well ;-)
+1 to the points about making it context sensitive.

+1 to the idea that tag clouds are a bad idea - they just aren't very meaningful.

A further point is to avoid using graphical banners, as many users confuse these with ads and will reflexively avoid them.