Negative interest rates - Monetary Policy
Real interest rates can be negative without a negative benchmark rate set by the central bank (inflation is higher than nominal interest rates) so in that sense we won't see any new effects.

Regarding the interest rate set by the Central Bank, it does decrease the real interest rate as the banks won't deposit money and have more incentive to give out loans but effectively it is not different from a zero rate (you can't force the bank not to sit on the cash) though I guess the bank has to pay the interest on the mandatory deposits.