I think that Wikipedia is excellent. To remind you some of it is written for experts in a specific area in math, thus if you do not understand it, it is natural.
You can use PlanetMath
Those, are the other two I like to use.
However, if you want to learn something. Go Here.
And type "MIT Math"
The search will show you a complete (or almost) complete list of lectures from MIT University on Differencial Equations and Linear Algebra.
You can also go here.
Then Click "Mathematical Methods for Engineers I and II".
Those are taped lectures.
I think it is a great idea to view them over the summer. It is just 50 minutes. If you watch a lecture once a day you finish the course in one month.