If you are a blogger, here are the must have plugins for your wordpress blog. These plugins are well ranked, updated frequently, and are very powerful.
1. All in One SEO Pack: Best SEO plugin for WordPress blogs you can set title to post, manage keywords, post discription.
2. Akismet: A popular wordpress plugin to check comments for spam. Its also must have plugin.
3. Contact Form 7: It helps you to design the forms for your blog, Its really easy to design a form with this plugin.
4. Google XML Sitemaps: Best plugin in wordpress to generate the Sitemaps for search engines. It can inform search engines for the new posts. Its also a must have for every wordpress blog.
6. More Link Modifier: This plugin allows you to easily modify the text for more link.
7. Post Randomizer: It allows you to randomize the post. This is helpful for those who dont update their blog daily. So everytime a user visits he find the new post.
8. SimpleAdPlacement: This plugin allow us to place ads after posts, or on the bottom of pages.
9. Keyword Statistics: This plugin suggest you, the keywords from you content, depending on the density of the content.
10. Sociable for WordPress 3.0: It add social network bookmarks to posts, pages and RSS feeds.
11. Subscribe Here Widget: This plugin displays feed icon and subscribe by email controls taken from feedburner.
12. TwitterPost: Whenever you post new article this plugin will post it to twitter.
13. WordPress.com Stats: It helps you to gather the stats of your blog, it includes page views, referer, stat chart, search query for your blog.
14. Social Slider: This plugin has large social network, which you can include in your blog, it will surely help you to get traffic from social network.
15. Post Page Associator: This plugin helps you to add the post to page in any wordpress blog.
16. Favicons: This plugin has large number of favicons, which can be added by simply selecting them. A very easy to use plugin.
17. Online Backup for WordPress: As the name suggest its the plugin for Backup purpose. It can backup database for your wordpress blog.
18. WP Responder: A plugin for auto responding, this works similar to Aweber
19. Infolinks Official Plugin: It will insert the infolink ads to your blog. First you will need to sign up and wait for approval. Its a substitute for other advertising network.