Having a collaborative blog has some challenges (organization is a big one) but using the right tools certainly helps.
Here is my list of the must have plugins for a collaborative blog.
I have this plugin on all of my blogs. It allows you to customize your login page with a logo (or any other picture for that matter). It completely erases any confusion one may have that they are attempting to login to the wrong website. Having a custom login page also makes your blog look professional.
The program is fairly easy to use and only requires minor tweaking.
Having multiple authors on one blog can get very confusing. Having a link checker is fantastic I receive emails when it detects broken link and I can fix them or delete the links. It is perfect in case one of the authors attached an incomplete link. I do not need to check every link during the editing process.
Again I have this plugin on all of my blogs. It is the perfect tool to see what new posts have been added, what has been scheduled and what has already been posted. Admin Post Color allows you to quickly determine the status of your blog with very little thinking.
Imsanity automatically takes large image uploads and scales them to a manageable size. Your guest posters may not care (or know) how much space an image takes on your server or that it will slow down your website. Using Imasanity allows you to save all of that time resizing images as it does all the work for you.