You must have seen many websites or blogs use this kind of sticky notification bar to display the important announcements or events on their blogs, so that their readers do not skip any important information.
LinkWithin is a very important widget for keeping your visitors to your blog because it shows the related posts at the end of each article. But if you are a blogspot user then it also shows the related post widget in the home page, and spoils the look of the home page. So, it matters a lot for your blog.
facebook "page like box" is a great widget for bloggers, which is used to promote your page. Having a fan page like box into your blog sidebar is a great practice to convert your visitors into loyal readers just by clicking the like button.
Blogs are all about content, but they’re also about engagement. The longer you can keep your reader on the page, the more invested he/she is in your blog. Blog widgets help do this by driving complementary content to your blog.
You might noticed on some site, there is some featured posts in post-footer with title "you might also like". Installing a widget that indexes and links historical posts from your blog makes them visible and easily accessible to readers who would not otherwise encounter them.
If you are a Blogger, I am sure you must be aware of advantage of having a Blog subscription box. Subscription box is a widget that simply allows the readers to subscribe to your blog, either via RSS or Email and to connect with you on various other social networking sites. So, it is an essential widget for every blog.
Blogger have many widgets its own but some are also from third party, But here is a list of some widgets from blogger and non-blogger owned widgets that should be on your blog for spicing up your blog, Many blogs have widgets that are to lazy to load, visitors hate that type of widgets, So let's start with some necessary widgets.