Pagebar adds a nice page bar for selecting indivual pages instead of just the previous or next page.
wpSEO plugin rewrites your blog title, META description and META keywords so these are more user and search engine friendly. (Page in German)
Amazon Book Picture allows you to include a simple tag with a books asin or isbn in your posts which is replaced by an image of that book from Amazon.
Fancy Pullquotes is a simple plugin for wordpress that allows you to effortlessy obtain a nice pull-quote of a specified text. The pull-quote will be formatted with a fancy nice style, exactly like one of those you can read on a regular magazine.
Alt to Legend is a plugin for WordPress 2.0 that allows you to display the ALT text of an image as its caption.
Updated Today displays a banner in the upper left corner of your blog when the date of the last post matches todays date, therefore telling the visitor that your blog has been updated today.