The Lite-version of Frame's own HTML-converter comes free with version 5, it's fully integrated into FrameMaker's menues.
The main drawback of the HTML-Lite converter is that the tag-mapping has to be done for every single file, and that this whole process is 100% GUI-driven, so one has to repeat the mapping-steps (too) many times. This makes it unusable for bigger documents. The full version of Frame's HTML-exporter does not have this disadvantage.