I think this might be one for Steve.
I found a data sheet and have attached it to this post. The basic information on the device is:
STP7NB80FP: STMicroelectronics N-channel "PowerMESH" MOSFET in TO-220FP (full plastic enclosed package)
Vdss(max): 800V
Id(max): 6.5A
Id (pulsed): 26A
Ptot at Tc=25C: 40W (derate 0.32W/C)
Rds(on): 1.2 ohms typical, 1.5 ohms maximum, at Vgs=10V
Vgs(max): ±30V
Vgs(th) 3V min; 4V typ; 5V max
Here are the suitable replacements I could find on Digikey:
1. Infineon SPA08N80C3:
http://www.digikey.com/product-detail/en/SPA08N80C3/SPA08N80C3IN-ND/593246
Equal or better in all important specifications except rated avalanche energy, single pulse - this one is rated for 340 mJ; the original is rated for 400 mJ. Lower Vgs(th) (3V typical vs. 4V typical) - probably not important.
2. STMicro STP10NK80ZFP (SuperMESH):
http://www.digikey.com/product-detail/en/STP10NK80ZFP/497-5972-5-ND/1578376
Equal or better in all important specifications except rated avalanche energy, single pulse - this one is rated for 290 mJ; the original is rated for 400 mJ. Capacitances and charge figures are all somewhat higher.
3. Fairchild FQPF9N90CT:
http://www.digikey.com/product-detail/en/FQPF9N90CT/FQPF9N90CT-ND/1955342
Also looks good. Higher avalanche specification. Slightly slower.
4. Rohm R8008ANX:
http://www.digikey.com/product-detail/en/R8008ANX/R8008ANX-ND/2791496
Looks fairly good apart from the avalanche energy specification - only 4.2 mJ compared with 400 mJ for the original part!
Many of the detailed specifications are beyond my understanding, and could be critical in your application (a switching power supply, I assume). Steve would be the best one to suggest the best alternative. I hope the research I've done will help him make a recommendation.