No post on videos would be complete without mentioning YouTube of course and Sal Kahn’s Khan Academy. A YouTube search on KS3 Algebra (KS3 is age 11-14) for example returns this list and for GeoGebra for example you will find a channel devoted to the software. For an alternative search of Khan Academy try Khan Instant. (Created by Ben Jacobson).
There are several excellent TED talks on Mathematics, these are all short talks and well worth listening to. See in particular Dan Meyer’s ‘Math class needs a makeover’. To learn more about TED see the ‘About TED‘ page.
This page on Mathematics for Students has an excellent collection of tutorial videos.