Well, assuming that you're already comfortable with Linux (I'd consider it a bad idea to use an OS you aren't familiar with, when you're playing around with money - imagine something "happens" while you're in the middle of a trade you need to watch), I'd suggest Interactive Brokers.
If you were looking for a platform which is not brokerage dependant I've got no ideas. But IB's TWS platform is Java-based, so it'll work on anything - even some toasters, if they've got the horsepower and a network card.
They also have a browser-based java client as well.. I've not used it but it has a very similar feel... that's actually how they demo the TWS client.
