The popular thing right now is writing websites using Ruby on Rails (a framework for writing web applications that has model-view-controller separation, templating, object-relational mapping to the database, etc. all designed with an eye towards agile development practices). It is supposed to be more concise than the Java options (my preferred language there with frameworks like Struts) and thus has advantages re maintenance, coding time, making alterations, etc..
I do have a Rails website I'm writing just for my own education, but I mostly use Ruby for system utility tasks. Putting logos on images a certain way, searching for duplicate files, spidering websites, altering large numbers of database values, running a set of IO benchmarks after tweaking a server setup, running some stats programs and XSL transforms on the output to generate a certain web page, etc..
Personally, I'd look at Perl before Ruby for that unless you are interested in Rails or like strong object-oriented programming support (the reason I'm using Ruby instead of Perl lately). Amusingly, however, the above code is a bad example of OOP since the most common way to work with files in Ruby uses a lot of class methods instead of objects and instance methods. I tend to write everything very procedural anyway, and only break things into objects when the logic starts getting too complex or I want to access the code from different programs without copying and pasting a lot of stuff that's better off being encapsulated.
I can't really make any comparisons re Matlab since I've only used Matlab for something beyond calculations once or twice (physics simulations mainly). Ruby probably lacks extensive scientific/numerical libraries since it was written by a professional programmer rather than in/for academia IIRC and has less mindshare than Perl to let them creep in.
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