thyroid.about.com is one i recommend to others because it's easy to understand and is written and maintained by a woman who is hypo and has had to educate herself about it.
there's also stopthethyroidmadness.com but it can seem a bit... um... over the top? there is good information there and i've found it very helpful, just take some of the "preaching" with a grain of salt.
i'm hypo and i'm on armour (synthroid didn't work for me) and it works great!