The best free spice is LTspice from Linear Technology. It is fast, free and not limited by circuit size. Visit www.linear.com to download a copy. There is a very nice Yahoo Group for LTspice -- lots of good advice.
It works under Windows and will work under wine for Linux.
Is a dificult question menawhile here he goes some sugestions:
For small/medium blocks that require high precision the best one in my opinion is eldo from mentor followed by Spectre from cadence and the for hspice from Synopsys.
If you want simulate large designers (toplevel circuits) that not require so much accuracy but speed the best one is hsim then nanosim boths from synopsys.
yeah,it depends on your exact design .
for small analog blocks , hspice,linux version or windows version ,is popularly used.
for toplevel or mixed-signal circuits, i recommend nanosim for your simulation.
an article said orcad 10 could support bsim 3.2. but up to what level it could support? by default, the model (like cmos) is level 1, and it could also handle level 3. even though orcad is not for ic design, but sometimes i still want to use it to simulate circuits with as accurate model as possible, give me somw clue, thanks.