Linkage's Exchange-Notes Connector synchronizes directories, databases, and lets users send mail to each other while preserving attachments and rich-text formatting. Microsoft has bought Linkage; they bundle it for free with Exchange 5.5 and above.
MESA's JumpStart currently has does MS Exchange public folder to Notes database synchronization. They plan to have application and mail synchronization in the future.
Casahl's RA.GroupwareLink synchronizes Notes databases and MS Exchange folders.
Binary Tree's MailPort for MS Exchange migrates, mail, C&S schedules, and contacts to Notes.