The common ones are:

Verilog-A
Verilog-AMS
VHDL-AMS

Google it, there is a bunch of stuff out there.
Jerry

Actually the MOST common one is MATLAB.. but this suffers GREATLY from the lack of ability to compare the results easily with the transistor level design..
Verilog-A/Verilog-AMS models can be simulated in the same environment as the transistor level design, but it still takes a knowledgeable person to do all the right simulations.. and compare the right things..
jbd