"like WoW" is the problem with your question. I estimate that Activision Blizzard is spending something like $300 million* a year just on development costs for WoW. There is nothing out there that is free which is having anywhere near that kind of effort spent on it.
So if you're asking "Are there free mmorpgs?" then the answer is yes: type "Free MMORPG" into Google and fill your boots.
But if you're asking "Is there a free game like WoW?" the answer is no.
* Just so you don't think I'm pulling these numbers out of the air, a little background: in June, Vivendi Games merged with Activision. While you can't tell how much Vivendi was spending on WoW annually from the pro forma financials they filed with the SEC, you can make an educated guess. The filing shows that 78% of Vivendi Games' revenues were coming from subscription and licensing and 0% of Activision's were in that category. The big difference between their two businesses is WoW, so it is not that much of a stretch to assume that those subscription revenues are coming from WoW and that it represents something like 3/4 of Vivendi Games' revenues pre-merger. In the quarter ended 3/31/08, Vivendi Games spent $102,716,000 on product development. If 75% of that is WoW, then they are spending about $75.3 million a quarter on WoW development, or $301 million a year. If WoW is wildly profitable and their other businesses only break even, subtracting all of the revenue from their other business straight from expenses (i.e. they make no profit there) leaves 73.3% of their total expenses, so 75% is not a bad guess.
And that is just development expenses. If you look at all expenses (including cost of sales, marketing, general, administrative and amortization/depreciation) and you use the same assumptions, they spend around $780 million a year to bring us WoW (they spent $259,769,000 on all expenses in that quarter, which annualizes to $1.04 billion).
Unless they have some other major piece of subscription income that I'm unaware of, the amount of money they are spending on WoW is massive. It is inconceivable that an ad supported game could come anywhere close to delivering the same product.