Exchange-traded funds, or ETFs, are simply diversified collections of stocks that are bought and sold as a single unit. All of the brokers I've covered here include ETF trading and most of them come without a commission.
The popularity of ETFs has skyrocketed in recent years, which means that most investment platforms have a pretty good handle on them. If you want to get started with ETF trading, you should be in good hands with any of the brokers covered here.