OpAmp with slightly negative supply for ADC input
Yes, though -500mV might be a bit on the high side, many chips specify -300mV max. This is more of a practical issue at elevated temperatures, but the abs. max. values typically don't distinguish.
In fact, as @DKNguyen says, you can buy chips that provide a drop-in solution, such as the LM7705.

I believe Maxim offers a similar part.