I use Interactive Brokers in the US, and have had a great experience with them. They're not exactly a mustachian choice though -- they primarily cater to active traders. This means while they have super cheap commissions, very competitive financing (can get margin loans through them at 1.6% right now) and a pretty good trading platform, they have fairly high activity minimums. On my account I basically end up paying a minimum of $20 a month between market data fees and minimum commission/activity fees.
Also, while, like I said I've had a great experience with them, since they cater to active traders they expect you to know what you're doing. I haven't heard very good things about their customer service, although in my limited interactions with it I haven't had any major problems.