I can vouch that some of the list from zerozero are not even close to a DX7 sound. Mr. Ray in particularly is a physical model of Rhodes-type electric pianos.

FM7 was probably the best known attempt to emulate a DX7 in software. It's now called FM8 and it's a bit pricey, but available from Native Instruments. Also, you might want to check out Native Instruments Komplete free player. I bet you that a DX7 electric piano is amongst the freebies. However, be prepared for a big download.

I think that in zerozero's list, Double 6 is probably going to be the closest thing for free. It's a true FM synthesis engine that has 32 DX7 patches. If the DX7 electric piano isn't one of them, I will be surprised.