These ETF Issuers Are Rising

BlackRock's

iShares. State Street's State Street Global Advisors

. Vanguard. The trio represents the dominant names in the ETF business. Some have even made the spurious accusation that the trio is

. To be sure, these firms are home to the bulk of ETF assets. Data from

show just how big iShares, SSgA and Vanguard are. Vanguard is number three in that group, but the firm had $233.3 billion in ETF assets under management as of October 5. That is more than triple the AUM of Invesco's

PowerShares unit, the fourth-largest ETF issuer. iShares had $531 billion in AUM and SSgA had $333.3 billion, according to the data, and it is numbers like that lead some to think the ETF business is closed to new entrants. On the contrary, there is room for more competitors and ETF inflows highlight as much. In 2006, there were just

. In other words, there is room at the ETF party for other fund sponsors to flourish. Here are some noteworthy examples.

WisdomTree is often referred to as the only publicly traded pure-play ETF sponsor. There is some debate around the

with naysayers saying the valuation assumes a rapid accumulation of assets that will be tough to meet. The company appears up to the task. As of May 21, WisdomTree had $14.7 billion in AUM,

. That number was $16.9 billion as of October 5. Some have said WisdomTree is a takeover target. Critics assert that a larger rival does not need to acquire WisdomTree because it can just copy the firm's ETF ideas. True, but that assertion is deeply flawed in that it does not consider the deep first-to-market advantage many WisdomTree ETFs have.

The world's 15th-largest asset manager was in the ETF game up until 2009, when the firm scrapped its lineup of low-asset products. Give Northern Trust credit for learning from its mistakes and persistence. Under the FlexShares name, Northern Trust's ETF push has been reborn. The firm introduced four new ETFs in late 2011 and those funds eclipsed $1 billion in combined assets under management in about 10 months. FlexShares ETFs had $816 million in AUM as of May 21, but the number swelled to almost $1.7 billion as of October 5. The FlexShares Morningstar Developed Markets ex-US Factor Tilt Index ETF

, one of the two newest FlexShares ETFs, is not even a month old and has already topped $10 million in AUM.

First Trust's move to almost $8.2 billion in AUM from $7.4 billion over the past five months is solid, though not jaw-dropping. Still, this is the ninth-largest ETF issuer and one that has made a connections with investors – institutional and retail – by offering different spins on common ETF themes. For example, First Trust's

are not carbon copies of traditional sector ETFs. There is nothing wrong with that as many of the AlphaDEX funds have outperformed traditional counterparts. Importantly, all of the AlphaDEX sector funds have more than $100 million in AUM. For more on ETFs, click

.