Imagery 3D is a freeware for converting images into 3D 3DMF formats.
Editor: If youre into 3D files and cant read the way too many formats available on the PC, Amiga, SGI and other platforms, Imagery 3D is in The Works to convert all these into QuickDraw 3D 3DMF formats.

This is a very early version so its very beta.. so beware... but it can support the following formats now:

Tested (more or less)
.3D2 Atari CyberCad 3D2
.3DS 3DStudio 3DS (colour supported)
.3DX Atari 3DX
.COB Caligari COB (colour supported)
.LWO Lightwave LWO (colour supported)
.NFF Neutral File Format (see note below)
.ENF Neutral File Format (see note below)
.OBJ Wavefront OBJ
.FIGURE Fractal Poser v2.0 脭figure脮 ACTR resources.
.TPL TPoly
.TNP TPoly
.BYU BYU Movie BYU

Untested (if you have a sample of these files, let me know...)
.SGO Silicon Graphics SGO
.POL InnovMetrics IMCompress Polygon file

Under development now
.DXF AutoCAD DXF R13
.ASC 3DStudio ASCII
.RIB Renderman RIB you can free download Imagery 3D 0.1.4 now.