I did some searching and this is what I came up with
IDX Microsoft Outlook Express file
IDX Index file (Microsoft Clip Gallery v. 1.x)
IDX Index file (Pro/Engineer)
IDX = Corel QuickFinder Information
IDX = FAX file
IDX = Header information (CMYK image) (Stork format)
IDX = ICQ index
IDX = Index file, used by many applications
IDX = LaTeX index
IDX = Microsoft Outlook Express file: mailbox
IDX = Misc
IDX = Relational database index (Microsoft FoxPro)
IDX = Symantec Q&A relational database index file
IDX = temporary Internet Mail file AOL