As a embedded systems programmer I always need to make lots of base conversions (dec to hex, hex to bin and so on...), and I must admit: Windows 7 calculator is a good calc, but too limited in my point of view. I work a lot with communications protocols and it`s common to need some base conversion in this field of knowledge.
I`m looking for a calculator software (not a hardware one), to help with base conversions, but it must also support scientific calc.
Since this subject is intended to help programmers, I did not ask this in SuperUser.com.
For quick stuff I use SpeedCrunch, which is a free, cross-platform calculator with a host of built-in functions, constants, and base conversions (use the Settings > Result Format menu for this).