WordPress plugins can really make your blog not only user friendly and SEO friendly but also author friendly. So this is what I will be going to tell you today, which plugins I like and use the most and can’t live without. Note that these are my personnel opinions on those plugins and I have never faced any problem with any of below listed plugins. So here they are :

1. One Click Installer: Install themes or plugins from your computer without uploading them. Also installs theme or plugin directly from the internet. More info at article on One Click Installer.
2. Google XML Sitemap: Automatically creates .xml and .xml.gz type sitemps and submits it to Google, Ask.com, Yahoo and MSN.
3. Sociable: Includes subscription and bookmarking links to many Social Bookmarking services after every post automatically.
4. WP Thread Comment: Allows your readers to reply to each others comments.
5. WordPress Database Backup: Creates backups for your site automatically and on demand. You can also choose to recieve the backup through email. Many other options also available.
6. All In One SEO Pack: SEO your blog completely. Very easy to use plugin that assigns keywords and description to every page of your blog. You can choose from various options.
7. AskApache Search Engine Verify: Automatically adds meta tags that Google and Yahoo provide for site verification. Just get the tags, copy them here and this will do the job all by itself.
8. CForms: Add contact form to your site. Very easy to use. Set your own looks and defaults for the contact form and then add it anywhere you want on your blog.
9. Google Analyticator: This one adds the javascript code that Google Analytics provide to your blog.
10. WordPress Download Monitor: Create download links for your stuff for others to download. Keep track of how many times the stuff has been downloaded.
11. Nofollow Free: Remove nofollow from your blog’s comments. Comes with many options and widgets.

And another plugin that I like is Headspace2.