Here are the gadgets or also called widgets that every blog should have
Popular Posts
This is very important for the blog owner and for the readers. You can use this to monitor the high performing posts when it comes to viewers and think of other posts related to these and write more of them. Readers eyes also can be easily caught by popular posts, I for myself everytime i land into a blog i will always look for the popular posts. Simply because we all want to see what most people view also.
Categories/Labels
It is also very wise to categorize your articles so that readers can choose on what certain category to look for and prevent them from leaving immediately just because they see unorganized articles. Specially for those blogs that have almost general topic, adding categories or labels gadget will help your blog a lot.
Sitemap/Table of Contents
You can put this as a new gadget or a static page instead. All reader will certainly want to see the old posts that are hard to find in the homepage. A sitemap is also easier to navigate and find a specific article because it is just a list of links without description or image so it is compact.
Search box
This is also a very important gadget for any blog with number of contents. It is much easier to type what you are looking for in search box than browsing all the articles and categories, that's for certain.
Numbered Page Navigation
Most blog templates specially the default one so not have page navigation installed. Numbered page navigation also helps your reader to browse articles specially the old ones also. Visitors who are returning to your blog and already read the latest articles might want to browse the older ones.
Social Sharing Buttons/Widgets
Social sharing buttons like facebook share, like, tweet are certainly also the most important things to add in your blog. Any blog today have them, these will definitely increase the traffic of your blog. As we know social networking sites specially facebook is second to google in terms of users. So don't disregard having these golds in your blog.
If you don't find these gadgets in your blog by default you can search the web for them. There are many professionals that create amazing gadgets for blogs.