I use 500ml decorative mason jars. Use the ones that do not have the shoulders since the shoulders prevent the liquid from expanding.
Also, Do not fill past the bottom thread, this allows the liquid to expand and not break the glass. http://www.bernardin.ca/pages/produc.../47.php?pid=39
I found the 500ml containers too small and am now trying out the 1.9L containers.
I am freezing 1.2L of 1.050 wort and then when I go to use it just dilute to 1.5 L before boiling. (so i get 1.5L of 1.040) I have standardized all my stepups to 1.5L