GNU Smalltalk is a really easy interpreter to use (I tried Squeak, but found it overly complex for simply learning Smalltalk).
I used it to start learning Smalltalk, and found the language itself to be quite fun (again, because of Squeak's complexity, I couldn't get into the language at all using it).
Performance is surprisingly fast - much of the functionality rivals more recent languages, most of the time out-pacing Ruby and even PHP, and even being competitive (speed-wise) against Python.