FLV MetaData Injector (FLVMDI) is a Win32 console (command line) application that can add 'onMetaData' AMF data to FLV files. (There's also a GUI frontend FLVMDIGUI). FLVMDI optionally saves an XML version of the injected data, and can add an additional string data or save the onMetaData data to an XML file. FLVMDI has some unique functionality, but it's not as vital now because flashcom 1.5.2+ and Flash Video Exporter 1.2+ adds the required metadata. (Unfortunately there's still a Gotcha with Sorenson Squeeze 4, also see Sorenson Squeeze (4.1) FLV Metadata upate...).
Flash 8 Video Encoder inserts the correct MetaData. Also Sorenson Squeeze 4.2 is released on Sept 20, 2005, hopefully they fixed the metadata issues.
FLVMDI is free to use, but you cannot distribute it or link directly to the download. FLVMDI is not exactly freeware, but only free to use and at your own risk that is. All rights other than granted here belong to Manitu Group.