I haven't seen a simple stand-alone converter but my Yamaha RX-V861 AV amp can convert between analogue formats. For example, I've sat playing my Xbox 360 connected via component video to the amp with it outputting S-video so I could hook my EyeTV up and use my MacBook Pro as a screen on my lap (my wife was watching Eastenders). This only works if the component output is set to 480i (ATV supports this) as the amp won't convert the field/frame structure. In the case of the Xbox 360 there is a handy switch on the side of the AV adapter which switches between HDTV and SDTV so that was all I needed to do.
Of course, the amp wasn't cheap but it does have lots of nice features like HDMI upscaling so all my video sources (SVHS, ATV, TiVo, Xbox 360, PS2, Wii, Blu-ray, LD/DVD) can all be sent to my HD projector over a single HDMI. The component to S-Video thing just happens to be a convenient extra feature.