Please follow the given link to be informed about the open source mixed circuit design. You can use winspice as low cost circuit simulator and use LASI as lay out tool. You need a technology file to make a successful design. and you can use the so called universal lambda based lay lout rules. With these tools we could design and simulate analog and mixed ic circuits: http://www.edaboard.com/thread174733.html
The well known EDA simulation tools based on Spice are
from Cadence (Spectre), Mentor (Eldo), Synopsys. All of them have also the complete flow from Schematic down to Mask Layout and their verification (Checks like DRC, ERC, LVS and parasitic extraction for postlayout simulation!).
There is a little less expensive solution from Tanner/Mentor (TSpice) including the whole flow from Schematic Entry to Sim, to Layout & Layout Verification.
But there is a lot of academic, thus freeware EDA tools!
Frrom Spain:
http://www.cnm.es/~pserra/apdk/
and their Spice Opus:
http://fides.fe.uni-lj.si/spice/
or X-Circuit http://opencircuitdesign.com/xcircuit/
NG Spice for Lnx & Win:
http://ngspice.sourceforge.net/download.html
Start reading also Wikipedia pages, there is a lot of introduction to the topic:
It is well-known that, you can use the SPICE ( HSPICE) model of the simulator for simulating of the your design, simply. Also, for drawing Layout of the your structure you can use the Cadence software.
Meanwhile, if you want to simulate your design just behavioral model, in that time the Simsides model simulator is good candidate for simulating, because in the mentioned simulator model, you can apply the effect of the parasitic resistors and capacitors, the effect of the noise and temperatures, and other parasitic parameters too, generally. Due to that these type simulation results are near to actual way, mostly.