The Aldiko (
www.aldiko.com) ebook reader is excellent and allows you to add your own content, but I believe it's only available in the Android market... so it doesn't fully answer the question of full syncing. Plus it only reads EPUB and PDF files. But it does allow you to add anything from your SD card, and you can have your Dropbox account store items on your SD card. In fact that might be the default for Dropbox.