I recently started making some order in the mass (and mess) of my e-books (mainly stuff from Project Gutenberg and similar, downloaded over the years since the early 90s - OK, that shows my age
)
So I did a little bit of research and tried basically every imaginable e-book manager out there (at least, the ones I managed to find - ebooks are not exactly mainstream yet).
Most of them were rather disappointing, until I stumbled upon Calibre.
Free, very complete, manages format conversions, works with Sony e-readers (I'm gonna receive a Sony PRS-505 next week)... it's definitely an excellent program!
My only gripe is with its user interface...
While from a "functional" standpoint it's workable, it's severely lacking in the "wow" department - it simply doesn't have the level of polish we're used to these days.
My wet dream would be an iTunes-like GUI for Calibre... that would be awesome!
But even without going so far, I think at least having the book covers displayed in a "virtual bookshelf" (like Mobipocket Reader or Sony eBook Library) would go a long way towards both pleasing the eye and furthering the diffusion of this wonderful program...