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.
If you think of sidebars in the case of blogs, it's a good guess the majority are set up by people who are not strong on UI, UX or usability. Often, they want everything they are linking to visible all the time or they simply do not know how to add context to their sidebar implementation.

With a platform like Wordpress for example, there are now plugins which will help you contextualize your widgets and customize them right down to the page/post level.

Overall, many sites now implement a global navigation horizontally in the header and then more granular navigation in the sidebar. My own preference is for sidebars to be contextually sensitive. It's generally not helpful when a user has drilled down from a homepage, to the specific post, or even lands directly on the post, to have the main content littered with mostly irrelevant meta data or secondary content.

Ideally, if the site is well-designed and a user is interested in exploring, the information scent presented in the global navigation (even if that appears persistently in a sidebar widget), then they'll discover the other information where it's contextually relevant.

Usually free-for-all over-widgetized sidebars are useless and some of the heat map types of applications will often show that a lot of that stuff will not get clicked much.