Egg cartons - and you can get little plastic eggs around Easter time to use as counters in them, in the meantime using whatever you have around or can find cheaply. You can also make pom pom balls of yard to use in them. You can even get very big cartons (25 holes or so) at restaurants that specialize in breakfasts - so they can make a 100 hole piece.
Coins, paper money (maybe play money would be a good idea), a toy cash register, acorns or eucalyptus pods, ice cream sticks with beans glued on them for counters, Hershey bars (see Hershey Bar Fractions ), raisins, measuring cups, crayons for drawing math pictures, heavy paper for making charts, sandbox to play in with all sorts of things, hopscotch, large unchokable sized beads (you can even make some special ones of your own with plasticene clay!), skipping, singing, rulers and yardsticks, string or ribbons, measuring tape, quilting pieces to go into your own quilt, clay...
Elizabeth Hainstock's books on Montessori in the home have several ideas for creating math manipulatives, as well as ideas on how to present them. There are also online sites that discuss how to make Montessori manipulatives -- some spiffy-up the Hainstock ideas, making them out of more durable material.
I can't find the link now, but somewhere there is a site for learning how to do algebra using legos!
I second (third?) all the previous ideas. Beans, buttons, blocks--all great for math. I like to play dice games, too, just making up rules. For instance, roll four dice then try to combine the numbers using each of the mathematical signs (+, -, x, /) to come up with the highest or lowest or whatever number you want. Whoever can get closest to whatever you're going for wins.
Thanks for the great suggestions everyone! I have books on hold now, and we have a whole stack of egg cartons that keep missing out on recycling day. I'm thinking maybe we'll have a math party to make a few sets/items & help introduce them, then leave them in the games closet to be pulled out when they are interested.
Lillian, that was a great article! I already had your site bookmarked actually, but I had no clue it was yours
I don't have any suggestions, just a little funny. My younger DD loves pinto beans and one day we used dry beans for counting. Of course, she had to try them Now, she always asks "are these eating beans or math beans" :LOL