For a selection of short videos for the Mathematics Classroom try the Class Clips from BBC Learning Zone. Choose Primary (5-11) or Secondary (11-18) a subject and a topic. Note that each video has several keywords making searching possible.
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.
I have created an Evernote shared notebook with links to some further sites which include Mathematics videos. (You do not need an Evernote account to use this notebook, though I strongly recommend it as an outstanding resource for capturing notes / ideas /anything! ). To navigate the notebook simply select the notes on the left, note that the url is given at the top of each note.