Squeak is by far the best and most complex Smalltalk implementation out there. It may not play well with other operating systems because it is an operating system by itself. It is also one of the most impressive development environments I ever seen. OK, the most impressive.
Freeing Squeak, freeing Smalltalk
The only thing that bothers me is that Squeak is not really free software (search for Squeak in that page). Fortunately, some people are working on making it proper free software. Hurrah! I hope the succeed soon!