I totally have to recommend a good, old-fashioned abacus. Especially if you can get your hands on a solid, real wood one. My husband thought I was a bit odd when he found it amoung our early Christmas presents, but I was thrilled!
The thing they're really good for is hands-on math. You can use those beads for just about any basic function, and your kid can actually see it without having a bunch of manipulatives rolling off the table and being chased by the dog...
You know how you can look at a group and just know there are five things in it? Levi is learning that, because the humble abacus lines up those little beads consistently. We've also already used it to practice counting by 2's, 5's, and 10's, as well as figuring out how old both the boys will be in 2 years (word problems!), and for subtraction, and for counting to 100.
Huzzah for the lowly abacus!