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Askimet for spam SEO Title Tag to make those title SEO friendly, it doesn't work with the latest version of WordPress (2.3) WPDB-Manger to help keep the DB backed up and safe
Speaking of Dagon, the Dagon Design Form Mailer is a great contact form. I can't tell you how many blogs I go to that don't have a way to contact the author...
As Fyrestorm mentioned, SEO Title Tag is awesome but it's not compatible with the new WordPress 2.3, currently I'm using Optimal Title for my title tags - http://elasticdog.com/2004/09/optimal-title/
Alex King has a great selection of plug-ins that he has made for WordPress, one of them I use is Popularity Contest which allows you to display and rank popular posts on your WordPress blog - http://alexking.org/projects/wordpress/
Share This is another great Alex King plug-in, it displays a link under each post that can pop up social bookmarking buttons or the option to email the post to a friend, again his plug-ins can be found at - http://alexking.org/projects/wordpress
Also, I didn't see if this was mentioned but I'm running Automatic WordPress Upgrade which does pretty much that the title says, it automatically or easily ( with a few clicks of the mouse ) upgrades your WordPress install to the latest version. I've had it turned off for a while when I was beta testing 2.3 but I'm using it now again... it's very slick. - http://techie-buzz.com/
SRG is great, and needed for stuff like payperpost. Askimet has blocked over 3000 spam comments on my blog in 8 months, I would just make love to it if I could.
I also use Pages LInk to, which adds functionality to link either a page or post to just about everything that can be linked. Useful when running WP as a CMS, or like me where I have all my top bar used for linking to my tokens of internet presence.
These are in my opinion, the must-have...there are much more anyway, but some of them were already posted here. This thread should be pinned It helps a lot the beginners and those lost in the information