In the past few months I’ve really ramped up using WordPress plugins, and they do some amazing things. All but two are free.
AddToAny is the widget at the end of each post which lets you share it on dozens of sites. I use it myself to to post to Twitter, Facebook, and Stumbleupon.
After the Deadline. Spell and grammar checker. Highly useful.
Akismet. Kills spam comments. Essential.
All in One SEO. Definitely improves visibility on search engines and does so completely ethically.
Backup Buddy. Backs up the database or the entire site and FTPs it to the Amazon S3 cloud. Fully automated. $45 and totally worth it.
FD Feedburner plugin. Takes all possible version of your main RSS and redirects them to Feedburner. This way someone subscribing to the RSS feed always gets Feedburner.
Pubsubhubbub. Instantly sends new posts to a cloud hub where they get picked up fast. With this, new posts on Polizeros literally appear in my Google Reader RSS feed 5 seconds later.
W3 Total Cache. The best cache around. 8 pages of configuration items (!) Be ready to spend some time learning it. Hint: If you are on a shared server, do not use database or object caching. It dramatically cut the load time here.
WordPress.com Stats. A great way to track how many people are on the site. Appears in the Dashboard.
WPTouch Pro. Displays the site in a mobile format for iPhone / iPad / Android / Blackberry. If using W3 Total Cache, you must allow and disallow various user agents in order for WPTouch Pro to work right. Both plugins explain what to do. $25 and also worth it.
What plugins do you use?