My favorites are M&M's Math and More M&M's Math but there are also Cheerios counting books, Hershey's books on fractions and more.
Count to 60 while brushing their teeth
Annalee and I take turns brushing her teeth and I count out loud to 60 for each of our turns. It's an easy way for both girls to understand the concept of one minute and they've picked up most of their higher numbers too.
Use Card Games
From War to Crazy 8's to Go Fish, these are fun and easy ways to get to know numbers and their relation to each other.
Double a recipe to teach multiplication, use measuring cups for fractions, divide your pie in 8 pieces and figure out how many each of you gets....
Build a Playhouse
Older kids can take part in building forts and playhouses. A nice side effect is that they'll painlessly pick up geometry while using measurements & angles. On a smaller scale, plan and build a lego fort.
Give Them Puzzles & Tanagrams
Young children learn valuable spacial lessons while putting together puzzles of all types. Tanagrams (geometric shapes that can be put together to create pictures and new shapes) are also great tools.
Get Some Cuisenaire Rods
These small wooden or plastic blocks have been used in Montessori classes since I was a kid and for good reason. They are a wonderful way to understand units, fractions & more, all while children play. You can find them at the Eta Cuisenaire site for about $15 or used from sites like E-Bay.
Let Them Earn Some Money
Even tiny tots can pick dandelions and turn them in for a penny apiece (5 cents for roots) or take part in extra credit chores for dimes and quarters.
Give an Abacus for a Present That Will Grow With Them
I got Victoria an inexpensive ($10 or so) wooden abacus when she was a toddler. She loved to move the brightly colored balls along the rods. Later when she started to learn numbers, we counted them. She's now getting to the stage where she'll understand tens and hundreds, and the abacus will help visualize these concepts. It's also a fun history lesson, using the world's oldest calculator to add, multiply and more.
Use poker chips
A neat homeschooling mom shared that she writes place numbers on poker chips in order to play with math concepts with her kids. Using white for ones, red for tens and blue for hundreds, she can easily help them visualize what 17 + 8 would be, how to find lowest common denominators and more.
Use Math Games
One highly recommended, kid approved book is Games for Math by Peggy Kaye. Check out homeschool catalogs to find some neat others.
Use what you have
To teach fractions, give a child some measuring cups and dried beans. For counting and adding, use hotwheels or dry cereal. Write numbers on dixie cups or egg cartons & fill them with the correct number of toys or buttons.
Do Connect the Dots
Paint by number, connect the dots and other "funbook" type sheets are great ways to learn early concepts while having fun.
Teach Them Some Tricks
Explore this wonderful site to learn lots of shortcuts and hints to pass on to your kids. http://personal.cfw.com/~clayford/
Play Dice Games
Kids can roll the dice & add the numbers, play "horses" or make up lots of games on their own.
Sing Songs & Memorize rhymes
Everything from "100 Bottles of Beer (pop?) on the wall" to "5 Little Monkeys Jumping on the Bed" teaches counting down and subtraction. Many jump rope songs involve numbers too, and kinesthetic learners (those who learn best while doing and moving) and auditory learners (those who learn best by hearing and saying things) will especially benefit from moving and singing while they learn.
Involve them in Daily Life
One of the principles of the "unschooling" style of home school is that kids will learn most of what they need to know through their own explorations and by taking part in daily family life. When they help figure out the best value of laundry detergent and factor in coupons, when they help plan and plant a garden, when they save up for a cool toy or play a football game, they're learning math. Math sneaks into all different other activities too- knitting, model building, sewing, board games, logic puzzles, nutritional information on the box of cereal.....