http://www.iunits.com: Details on ETFs offered in Canada by Barclays Global Investors, including the popular i60.
http://www.ishares.com: Barclays' site for its 60-odd U.S. funds, called iShares.
http://www.holdrs.com: Details on the 11 holding company depositary receipts created by Merrill Lynch.
http://www.streettracks.com: Details on new ETFs run by State Street Global Advisors.
http://www.amex.com: Information on all ETFs listed on Amex, including top holdings and MERs, plus quotes and graphing tools. A great site.
http://www.spdrindex.com: Data on select sector SPDRs.
http://www.indexfunds.com: News and views on U.S. ETFs and index funds in general.