Autotagger will help you create relevant tags for each post you make, and will create an inbound link from Wordpress.org to your blog. This is good from an SEO perspective.
Sociable
This popular free plugin adds social networking icon links to your posts,which makes it easy for your readers to share your posts across major social networking sites.
Wordpress.com Stats
This is a must have free plugin that gives you valuable information about the traffic your blog receives.
VideoPress.com
This is a commercially available plugin that lets you display video in various ways on your blog. It is especially useful if you plan to show long videos on your site.
Askimet (Spam)
This is another “must have” free plugin that identifies and intercepts spam comments on your blog. Without it you"ll be constantly cleaning up unwanted spam.
All in One SEO Plugin
This is another very popular free plugin designed to help you optimize your blog from an SEO perspective.
As I mentioned, there are thousands of other plugins. Choose some that sound interesting to you and remember not to go overboard installing too many at once.