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: 
Be careful about what you put in the sidebar. Most often people just throw stuff in there that they are used to seeing. Drop the archive links (who browses by date?), especially if it's a calendar, drop the tag cloud, etc.

Put things that people are likely to click on, a list of recent posts, if they liked your post, they will probably want more like it (a list of related posts is even better, but usually put below the post). A search box (if it's not already somewhere else) is necessary. Then give people options for being notified of new posts from you, an RSS feed, an email subscription box, etc.

You can also put links to other things you do that people visiting your blog might be interested in (other blogs, link to communities you participate in, etc.).

Usually, just thinking about what *you* really use in a sidebar is enough to know if a widget makes sense.
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.