IMAPCheck is a bundle for Mail.app that fixes an issue with IMAP support. Namely, when using server side rules to re-route email into folders, Mail.app wouldn't see the new mail until you actually clicked on the subfolder. This bundle changes the way Mail checks for new email on IMAP accounts by forcing it to do a full sync every time. Note that this could increase the amount of traffic going between you and your IMAP server significantly.
Note from the Developer: This plugin is no longer needed in 10.5 and is no longer supported.