b2evolution blogs are installed with a default selection of Widgets. For example, the sidebar of this blog includes widgets like a calendar, a search field, a list of categories, a list of XML feeds, etc.
You can add, remove and reorder widgets from the Blog Settings tab in the admin interface.
Note: in order to be displayed, widgets are placed in containers. Each container appears in a specific place in an evoskin. If you change your blog skin, the new skin may not use the same containers as the previous one. Make sure you place your widgets in containers that exist in the specific skin you are using.