I previously used the Database Backup plugin but now I use the WP DB Manager by Lester Chan. This plugin not only has the option for doing regular backups but you can also optimize and repair your db,empty/drop tables right from your WP admin panel. You can even run SQL queries and restore the backups you made. It's a must-have for any blogger running WordPress. For keeping track of my blog stats, I use WP Slimstats which uses up less db space than WP Shortstats. There's also another plugin that you can try, FireStats.
I'm surprised no-one has mentioned 'subscribe to comments'. For me, this is the best way to get your commenters returning to your blog too extend the discussion. Sure, there's coComment, but this is simpler. Also, I prefer 'similar posts' rather than 'related posts', purely because it's easier to set up, and I was too much of an idiot to get UTW configured properly (I guess tagging can be a chore too).
Someone said All in One SEO doesn't work. I don't know, I haven't isntalled it yet, but I've just read this post @ Smashing Magazine and I think I'm gonna give wpSEO a try
I'll refer to my list in another thread: http://forums.digitalpoint.com/showpost.php?p=4138003&postcount=14
I used to use SEO Title Tag but found that the All In One SEO plugin does what the title tag plugin plus more.