I just ordered the PRS-300, so I picked up Calibre to use as a management/conversion tool. So far so good, so I just wanted to say THANKS KOVID!
For users new to Calibre, don't get scared off by this thread. The program is very simple to use, just open your e-book (whatever file) and it will copy it to the Calibre library. From here it is very simple to convert the file into different types (.pdf to .epub, for example), and edit the metadata (image cover, title, author, etc.).
Unless you're a hardcore e-book collector and need to set up your own system you'll be fine. Also VERY helpful for folks new to collecting e-books.