FileInfo.com - The File Extensions Resource

.ABBU File Extension

File Type	Address Book Backup
File Format	File
Common?	No
File Description	Backup file exported from the Apple Address Book (included with Mac OS X); used to restore the default address book if needed
Program(s) that open abbu files	Mac: Apple Address Book

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/abbu