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

And MathWorld

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.

Click Math.

Click Graduate.

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.