Here are a few WordPress plugins you may find helpful in giving your WordPress blog some added functionality:
RelatedPosts provides a variety of options for displaying posts that are related through tags.
TextMe sends a carbon-copy of all admin emails sent via SMS to your cell phone.
Video Embedder makes it easy to embed videos from various sources, such as YouTube and Google Video.
Minimalist Gallery allows you to display an image gallery in posts or pages.
Social Dropdown displays several social bookmarking options in a dropdown.
Theme Test Drive lets you test out a theme while your users view only the default theme.
WP-Members restricts WordPress content to be viewable by registered site members.
Simple Feed Copyright will add a simple copyright notice at end of full text articles in your feed.
Collapsible Categories Widget allows for collapsible category lists in the form of a widget.