Here are the plugins I recommend for use on a WordPress blog.
Most (not all) of these plugins are generic, meaning that they are not bound to a specific type of blog and may (or should) be used regardless of what you write.
All of these plugins are top quality (IMHO) and provide an invaluable service for any blogger. I wholeheartedly recommend that you use and support them. I think that all of these should be apart of a standard WordPress installation.
Everyone knows this one but it deserves mention. It prevents content spam. I’ve used it for about two years and so far it did  not miss! I believe it now comes bundled with WordPress out-of-the-box.
Prevents link and robot spam. Again, very good service repelling e-mail address harvesters, spam bots and all kinds of evil insects.
An excellent alliance between WordPress and the PHPIDS project. It monitors your website for a very wide range of security attacks and does it very well.
4. WP Total Cache
This plugin enhances the performance of your blog and it seems to me, after trying out wp-cache and wp-super -cache that this is the best of the lot. It can even use a memcached server (if you have one).
5. FlagrantDisregard FeedBurner
This plugin provides the fundamental functionality of redirecting you blog feeds to FeedBurner, so you can track your subscribers properly.
6. WordPress Stats
Again, a fundamental feature: basic statistical tracking of your website right in the dashboard.
I have yet to see a better syntax highlighting plugin than this one. Full-featured, configurable, knows any language in the book and very easy to use. Mix in a good mono-spaced font and your set!
Ok so maybe there are more, but these, I find myself installing constantly on all blogs that fall into my hands (except number 7 – CodeColorer – since it is only meant for posting source code).
Enjoy… and contribute if you like!