This is a rolling list that is always being updated. Make suggestions on plugins in the comments and I’ll add them to the list.
SEO
SEO Friendly Images – adds ALT tags to all images automatically if you don’t (helps get more Google image searches)
SEO Searchterms Tagging 2 – adds recent search engine phrases to bottom of post automatically
Platinum SEO Pack – you can change the title tags/descriptions for the entire website, or post. Also makes URL’s canonical (ensures
No Self Pings – keeps wordpress from adding putting pingback comments
BWP Google XML Sitemaps – builds sitemaps that submit to search engines (faster search engine indexing). Works on multi-site installations
WordPress Related Posts – displays suggested posts at bottom of post based on content
Simple Tags – suggests tags in post editor based on content
DoFollow – adds ‘dofollow’ to comment links. This spreads backlinks to commentors (encourages commenting)
Broken Link Checker – checks for broken links and videos (Youtube, Vimeo, etc…) weekly
Random Posts Widget – displays random posts in widget area to encourage reading other posts
Target Blank In Posts And Comments – Automatically opens external links in new windows from posts and comments so visitors always stay on your domain
Security
Login Lockdown – limits wrong login attempts
Akismet – removes spam
Conditional CAPTCHA for WordPress – requires captcha entry for comments. Very good at stopping bots
WP Security Scan – checks your WordPress installation for database, permission, and other vulnerabilities. Hides things like WordPress version and protects login screen.
Google Analytics Dashboard – shows traffic on each post from dashboard (does not install Google Analytics into website)
Google Analytics – simple plugin to install your google analytics tracking code
Posts
Thumbnail For Excerpts – shortens blog page posts and automatically sets first image as thumbnail
Lightbox Gallery – makes image galleries pop up with lightbox
GetSocial – Social networking site scrolling box (like Mashable’s)
Facebook Like – Displays the like button for that post at the top or bottom of your post
WP Greet Box – Creates a box at the beginning of your post encouraging the visitor to look around based on which website referred them. As of this writing, it was missing from WordPress.org, so you can download WP Greet Box here
Contact
Contact Form 7 – Make contact forms easy and flexible