Steven Strogatz's series on Math: From Basic To Baffling is chugging along nicely. It presents interesting mathematical concepts, and appeals to your intuition in showing how they arise and why they work. It has an entertaining, easy-going pace, established right in the first article: From Fish To Infinity.
I think the best, so far, is the second article -- Rock Groups, which uses arrangements of rocks to prove theorems involving natural numbers. Since then, it has moved on to subtraction and negative numbers, and division and rational numbers.
This week, the series takes baby steps in basic algebra,with The Joy of x.
Working with formulas ... is a bit like art and science. Instead of dwelling on a particular x, you’re manipulating and massaging relationships that continue to hold, even as the numbers in them change. These changing numbers are called “variables,” and they are what truly distinguishes algebra from arithmetic.
The formulas in question might express elegant patterns about numbers for their own sake. This is where algebra meets art. Or they might express relationships between numbers in the real world, as they do in the laws of nature for falling objects or planetary orbits or genetic frequencies in a population. This is where algebra meets science.
This division of algebra into two grand activities is not standard (in fact, I just made it up), but it seems to work pretty well. [...]