monline

"IMAPSize is a freeware Windows application that allows you to quickly and efficiently manage your IMAP mail accounts. It is not another mail client. The main purpose of IMAPSize is to locate space consuming folders and allow you to manage them, hence putting your storage quota under control. To accomplish this task IMAPSize offers a variety of useful (and some unique) operations."

I used it in Ubuntu 9.10 (Karmic) with Wine v1.2 and worked great. I wanted to find the folders consuming the most storage space in an IMAP account. The backup/restore (synchronization) functionality has a small bug, but there's another tool (OfflineIMAP) that can be used for this task. You can read about both tools on David's blog.