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 ;-)
I'd say that the cleaner the side bar is - the better. 

My personal preference is to have there some info about the blog or the blogger, plus maybe a blog roll, which most people expect in the side bar.

I think tag clouds are usually useless and I'll explain:

* The context is too wide (the whole blog). It's much more effective to use the tags in each post to follow through to the topics that interest you.
* Often the most prominent word is the topic of the blog and since it appears on almost all the posts, it becomes redundant.
* Sometimes tags are abused for SEO purposes, so some might have weak connections to the actual content.