Howdy,
Here are some sites that I have used to complement my textbooks and such in the past.
For K-12 Math help:
http://mathforum.org/dr.math/This site is run by math wiz volunteers from around the globe. They explain the concepts quite nicely and fairly easy to understand for their respective levels. Topics include basic addition and subtraction, calculus and trigonometry, and pretty much everything in between. The site is also easily navigable and very easy on the eyes (the only advertisements I have seen so far are for related math textbooks, and those are only few on the main page). This global group will also answer generalized homework questions sent by email (within a business day or two, generally). Highly recommended!
For unit conversion:
http://www.onlineconversion.com/This site is extremely helpful for those who want to double check their conversions between units. The complete list of conversion topics is too long to list here, but it includes trigonometry, astronomy, and physics unit conversions, to name just a few. The possibility of conversions does not just encompass one country, but a multitude. (I did not know a "candy" is an Indian unit of measurement, and a "carga" is a Mexican unit of measurement, but I can convert between the two if I so choose. Not bad.) With all units of measurement you could think of, and tons more, this site is extremely helpful.
I hope this helps someone, and I'll post more high quality, free sites here for homework help as I come across them. Have a great day!