I am also looking for matlab replacement, although matlab is `de-facto' standard for signal processing.
As far as I know there are several (free) alternatives:
Python (scipy+numpy), some people in our university was productive using this language as a glue to fortran routines. Although I tried to use it several times I never progressed far enough. I got frustrated, when python interpreter didn't like my tabs for code alignment.
And there is even Ruby for scientists.
My first computer language was perl, so I might try it for signal processing.
I would like to hear about the succesful transitions from matlab to anything else. Especially in the areas of signal/image processing, communications.