Opera Widget just posted an updated version of their ebook reader widget. You can download it here. It's a very basic reader; it doesn't have dictionary, highlighting, annotation, or any other advanced features. Epub is the only supported format, and there is no DRM support.
I tried it on my laptop, and it was a good reading experience. I think I might switch over from the calibre ebook viewer. I'd love it if someone would try this widget on a mobile device and tell me how well it works on a small screen.