What do you think most popular usage of FPGA?

I doubt it. Not many people make ASICS, and while they make use lots of FPGAs for a single ASIC prototype, they won't use a continuous stream of FPGAs.

Lots of companies make industrial equipment that sell 100s or 1000s of units a year that each have an FPGA. FPGAs can often be used when you want an initial release and add features over time, or be able to customise units to customer demands.

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Would be interesting to know. I'll bet some folks at Altera know the answer and could describe the market in detail. Like gj_lesson said, there are plenty of audio/video applications (I'd add networking), and I'll bet there are a few small FPGA's in some consumer products too. The real money might be in the expensive FPGA's in RADAR/Satellite or military products.

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There is massive crossover between the areas, but FPGAs cover massive data processing. Basically anything involving real time packet switching at 1G+ speeds will have an FPGA or an ASIC doing the work. ASICs will only be in place in the more mass produced products (Like Ethernet switches) while lower volume products will have FPGAs (and probably more than 1 per box). In addition, anything that needs high speed transceivers and mux/demux will be in an FPGAs.

Basically, anything with any high bandwidth requirement - you're going to need an FPGA.