I use WordPress every day and find that some plug-ins are just better than others. A robust plugin will be compatible with most WordPress versions or at least degrade gracefully when others will break the design or even show a blank page.
Here are a few plug-ins I use regularly (listed from A to Z):
Anti-spam plugin from the guys at Automattic. Included with WordPress by default.
- All in One SEO Pack
Out-of-the-box SEO for your WordPress blog. For when your theme needs further optimization.
Recently turned open source. Easily create multi-page form-to-emails while capturing it in the database as well.
- Google Analyticator
There are plenty of good Google Analytics plug-ins availible. This one creates a dashboard widget with a graph and popular keywords.
- Google Analytics Dashboard
Google Analytics graph integration. Shows a mini graph along side every post and page.
- Google XML Sitemaps
Generates a special XML sitemap which will help search engines like Google, Yahoo, Bing and Ask.com to better index your blog.
- Twitter Tools
A complete integration between your WordPress blog and Twitter. Bring your tweets into your blog and pass your blog posts to Twitter. Show your tweets in your sidebar, and post tweets from your WordPress admin.
Adds a more advanced paging navigation to your WordPress blog.
- WP Security Scan
Perform security scan of WordPress installation and helps reduces the risk of being hacked.
Which plug-ins you will use depends on what you want to do, but these are the ones I most frequently use. Post your favourite plug-ins in the comments below.