Mods please remove if this thread is too old to add to but everyone is suggesting simple green, detol, fairy power (NAIL POLISH REMOVER?!? no no no) etc etc which work and are great but they are expensive and they take ages. No one has mentioned what is easily the best one:
Methylated Spirit. This stuff:
It's dirt cheap and does the job in a fraction of the time of the rest. It will strip a model back to grey plastic inminutes if you peridocially scrub with a tooth brush, less scrubbing required if you buy a cheapy ultrasonic cleaner. You can leave a plastic model in it for days and nothing bad will happen. The only downside is that while it does work on resin I wouldn't use it, it will soften resin if you leave it in for too long.
I have found Nitromors marginally better on metal figures but it is very expensive and can be compensated by leaving it in Methylated spirits longer. (or use Nail polish remover

)