You're right that BCD is very common for calculators. BCD was also commonly used in microcomputers, since you save all the binary-to-ASCII code for I/O. This is why x86 has a bunch of BCD instructions like AAA (ASCII adjust after addition), which was important enough to be a single-byte opcode.
The 6502 is notable for its highly-efficient and patented (https://www.google.com/patents/US3991307) decimal arithmetic mode. I'll write up its interesting circuits sometime. One consequence of the patent is the processor in the NES video game is a 6502 clone that lacks decimal mode.
I had thought the 2A03 didn't have BCD to save silicon-- but apparently they just crudely disabled it by removing 5 transistors[1]! Would Ricoh's second-source 6502 license have been more expensive if they included BCD?
The 6502 is notable for its highly-efficient and patented (https://www.google.com/patents/US3991307) decimal arithmetic mode. I'll write up its interesting circuits sometime. One consequence of the patent is the processor in the NES video game is a 6502 clone that lacks decimal mode.