1. Admin Menu – add admin menu links to the top of your blog
  2. Advanced WYSIWYG Editor -more styling options to the WYSIWYG editor
  3. Akismet -fight spam
  4. Contact Form – allows site visitors to contact you
  5. – statistics plugin for WordPress
  6. Google Analytics – adds Google analytics to your blog
  7. Google Sitemaps – sitemap generator – better search engine indexing of your blog
  8. Math Comment Spam Protection – extra protection against spam comments
  9. Related Posts -returns a list of the related entries on your blog
  10. Sidebar Widgets – adds “Sidebar Widgets” panel under Presentation menu
  11. Simple Trackback Validation – eliminates spam trackbacks
  12. SpotMilk – enhances and beautifies your admin theme
  13. WordPress Database Backup – backs up your blog
  14. WordPress Reports -Generates reports from Google Analytics and Feedburner data
  15. Comment Quicktags + -Inserts a quicktag toolbar on the blog comment form

Avoid stuffing your blog with unnecessary plugins.Use only what you really need.

I still have to discover lots of useful wordpress plugins so the list will stay a Top 15 for the moment and I will try updating it as soon as I can.

Update. A friend of mine,Shiva,made a list too with the wordpress plugins he found very useful.Thanks Shiva.
So here it is:1.Ultimate Tag Warrior
Allows tagging posts in a non-external-system dependent way; with a righteous data structure for advanced tagging-mayhem.
Get it here
2. Spam Karma 2
One of the best spam killing plugin for wordpress. I don’t use akismet as many times it considers genuine commentor’s comment as spam so I use spam karma 2 and it rulez.
Get it here
3. Subscribe To Comments
Allows readers to recieve notifications of new comments that are posted to an entry
Get it here
4. Gravatar for wordpress
This plugin allows you to generate a gravatar URL complete with rating, size, default, and border options.
Get it here
5. Browser Sniff
Detects web browser type and operating system of your commenter.
Get it here
6. Brian’s Latest Comments
This shows an overview of the recently active articles and the last people to comment on them.
Get it here
7. Bannage
Bans commentors by IP, username, email, or URL.
Get it here
8. Popularity Contest
This will enable ranking of your posts by popularity; using the behavior of your visitors to determine each post’s popularity. You set a value (or use the default value) for every post view, comment, etc. and the popularity of your posts is calculated based on those values. Once you have activated the plugin, you can configure the Popularity Values and View Reports.
Get it here
9. Show Top Commentators
Encourage more feedback and discussion from readers, by rewarding them every time they post a comment! Readers with the most comments are displayed on your Wordpress blog, with their names (linked to their website if they provided one).
Get it here
10. Sociable
Automatically add links on your posts to popular social bookmarking sites.
Get it here
11. WP-UserOnline
Adds A Useronline Feature To WordPress
Get it here
12. Landing sites
When visitors is referred to your site from a search engine, they are definitely looking for something specific – often they just roughly check the page they land on and then closes the window if what they are looking for isn’t there. Why not help them by showing them related posts to their search on your blog? This plugin/guide lets you do that, works with a long list of search engines!
Get it here