According to
this announcement over at the Mobipocket forums, the "professional" version of Mobipocket Reader is now available for free (previously $19.95). Unlike the standard version, pro includes connectivity support for
eNews, highlighting, page autoscroll, free drawing, annotations, custom linking, and more. In addition, the latest version 5.1 adds support for importing Microsoft Office, Adobe PDF, text and HTML documents, and support to synchronize bookmarks, annotations, reading lists, and book metadata between devices.
You can download Mobipocket Reader Pro 5.1
from here.