"Americans seem to be more isolated than their counterparts overseas, although people are now changing their thinking," he said. "Going forward, if you want to make a good return, you're going to have to consider overseas markets."
Mr. Kearns at NeoNet, which is a unit of Stockholm-based NeoNet AB, said upward of 70% of his firm's business is from European clients trading in other European markets, but the percentage of U.S. clients trading internationally has increased.
"There's been a significant pickup in U.S. trading into Europe," he said, adding that improved market conditions have led more U.S. investors to move more investments outside U.S. borders. U.S. stock valuations have also played a role.
"With valuations so rich in U.S. markets, investors are looking globally for value, so we've noticed some value fund players really going to Europe and looking over there for opportunities," he said.
NeoNet, which currently offers access to the New York Stock Exchange, Nasdaq, the American Stock Exchange and markets in Amsterdam, Brussels, Copenhagen, Frankfurt, Helsinki, Lisbon, London, Madrid, Milan, Paris, Oslo, Stockholm and Switzerland, is considering expanding to Asian markets.
Instinet offers access to 40 markets around the globe, including markets in Asia, according to Mr. Plunkett.
"Theoretically you can be trading 24 hours a day," he said, adding that the firm has trading desks in Hong Kong, London and Tokyo — as well as New York — for investors who want their orders handled by traders rather than electronically.
Triglobal Wealth Management, Montreal, a unit of Canadian financial services firm Triglobal Group, uses Interactive Brokers to trade U.S. stocks, but Mario Bright, Triglobal president, said access to European markets provides new opportunities.
"Interactive Brokers' expanding base of products to include European markets opens the door to opportunities we never had before," he said.
"These markets were accessible to us before, but not with the ease of accessibility offered by Interactive Brokers."
Indeed, cross-border trading is nothing new, but investors using services like Interactive Brokers, Instinet and NeoNet say using global broker-dealers can be difficult.
"The problem with the major Wall Street firms is that they are highly departmentalized," Mr. Bright said. "Execution, accounting, back office and compliance are all different departments that we would need to deal with on a daily basis."
At Interactive Brokers, he said, all those functions go through a single point of contact, making the cost lower.
Other global investors, like Kristian Wiermyhr, who runs the hedge fund Predator Global Macro Fund out of Oslo, Norway, pointed to potential conflicts of interests that major broker-dealers might have.
"Some customers want to talk to a broker for comfort or advice" but risk giving the broker too much information, said Mr. Wiermyhr, who trades European futures and options through Interactive Brokers. "There are not many brokers worth talking to. It's better to be on your own."
Mr. Kearns at NeoNet said many buyside institutional investors are concerned about conflicts of interests, which gives his firm a leg up: "The fact we're neutral and we have no axe to grind is a big plus."