GoodReader can now sync with the Dropbox, iDisk and SugarSync services, plus any WebDAV or FTP server. It handles a bunch of different types of documents (pdf, .txt, .doc, html, images, audio/video files, and iWork files). Setting up sync is as simple as adding a location and pressing the sync button. If you’re having trouble keeping the most recent copies of your files updated, GoodReader is a great way to solve that problem.
The update is available now in the iTunes App Store. It’s free for current owners of the app.
GoodReader | iTunes App Store via TUAW
Worth noting that ReaddleDocs also has 2-way sync with dropbox which works really well. It also has a better UI than GoodReader, and a faster PDF engine. It has fewer annotation facilities though, and is more expensive. I have both, but find myself using ReaddleDocs for its speed and nice interface.