I use more than just ten plugins on each of my own wordpress blogs, but if I had to choose just ten, then these would definitely be it.

1. Askimet
   Checks your comments to see if they’re spam and separates the ones it thinks are, so that you can delete them easily.
2. CommentLuv
   Checks commenters’ blogs for their latest post and adds it as a link under their comment. Latest version allows commenters to choose which of their posts to use.
3. Google XML Sitemaps
   Creates am XML sitemap (whatevertheheckthatis) for you to send to the Google people because they want one to list your site properly.
4. All-In-One SEO Pack
   Automatically optimises your blog for search engines.
5. MaxBlogPress Ping Optimizer
   Stops wordpress from sending nuisance pings to sites like Technorati when you change an already-published post.
   Also ensures wordpress pings when your scheduled posts are published, instead of at the time you schedule them.
6. WordPress Database Backup
   Automatically backs up your wp database on a weekly basis and emails a copy to you.
7. WordPress.com Stats
   Love this one. -Actually, I heart-this.
   Really useful information, like what google searches readers used to find your blog, which links in your blog they clicked on when they left it, and of course how many visitors your blog has had (by day, week & month).
8. DoFollow
   Not all bloggers know that Google doesn’t view comment links as valid links towards their PageRank. This plugin removes the “no-follow” attribute from comments and your commenters will love you.
   Compliments CommentLuv very nicely.
9. WP Supercache
   Speeds up your blogs’ loading time by creating a html page for regular readers.
10. Twitter for WordPress
    And finally, a little fun. Add your latest few tweets to your sidebar, to make your blog look a little more lived-in.