There's so many plugins it's hard to think that there's not a plugin for anything you need. What plugins would you like to be made that are not made already?
One that creates meta keywords tags and that same info is used to create technorati tags in the post and that same info is also used to submit that blog post to del.icio.us.
It would be a great tool for blogs. The meta keywords would help to keep them unique for each post and can be helpful for whatever powers that be still use meta tags (the extension should also include a mechanism for meta description tag as well for completeness). Those same keywords can be easily inserted into a post as Technorati tags. Then it is just a matter of using the del.icio.us API to use those same keywords when submitting the page to a user's profile. All three are useful for a blog's promotion and all somewhat related so I think one extension that does it all would be very popular (I know I'd tell a lot of people about it. Heck, whenever I installed a blog I would probably make it part of the default install).