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: 
Sidebars are great! you just have to use the right widgets.

I use sidebars for sub navigation, or to promote certain pages on a site, or to promote an activity (like register here, or buy now).

Many people use ready made templates and don't know how to setup the sidebar widgets, but those who do get a lot of use out of them.
Avoid using chat boxes on the sidebar of your blogs. An example is the Cbox, etc. This will only take away comments on your blog posts and is a magnet for spam messages.