Based on a tip on Stack Overflow, I just discovered Scilab, a FOSS (free/open source software) equivalent to Matlab. Then I discovered that I had actually only rediscovered it: Scilab was already installed on my computer. Maybe my scientific software collection needs a cleanup.
Anyway, as I mentioned, Scilab aims to do basically what Matlab does. It's an admirable goal, but I'm not such a fan of the program itself, for a couple of reasons: first, at least on Linux, it looks ugly, and more importantly, the GUI seems to lack some of the basic functionality that people tend to expect these days, like scrolling. If you're a fan of xterm then perhaps Scilab is for you, but otherwise it's kind of inconvenient to use. For the rest of us, the need for a Matlab equivalent is pretty well filled by Python with Numpy/Scipy and matplotlib, plus your terminal of choice. Python also has the benefit of being a general-purpose programming language, so once you learn it for numerical computation, you can do all sorts of other cool things with it.
A very pertinent warning from the How-To Geek:
If you Google (or Bing) for any number of open source applications, the first result will be an ad at the top that takes you somewhere other than the real site. Here’s just a few of the applications we’ve noticed this happening on, but there’s a ton of others.
As I write this, the problem seems to have been fixed on Google, at least — Google searches return the proper site as the top result. (Or there was no problem, but I wouldn't expect howtogeek.com to make this stuff up) Still, I added in the links above in case you're looking for one of the programs mentioned. In general, whenever you're downloading a program, do some research and make sure you're getting it from the original source! Search engine gaming is a big business these days and a little caution goes a long way.
P.S. Although I don't want to be a hardcore Linux evangelist, this is one of the benefits of running Linux: all your programs are installed through a package manager which verifies these things for you.