BBC You Tube page (other non-Who stuff here also)
Various You Tube VIdeos:
Series 4 Trailer. Although the BBC have this you can't watch it outside the UK. This one you can.
Some of my favourite episodes (links to Wiki page for full story and spoilers):
The Tomb of the Cybermen
The Terror of the Autons - The Master (Roger Delgado) and some Autons!
Day of the DaleksSilver Nemesis
The Sea Devils - The Master again (and some Sea Devils)
The Three Doctors - Meet Omega, a Time Lord.
Death to the Daleks
Planet of the Spiders - Spiders, Time Lords, UNIT and more!
Genesis of the Daleks - where it all began
Revenge of the Cybermen
Pyramids of Mars
The Brain of Morbius
The Deadly Assassin - a trip to Gallifrey
The Robots of Death
The Invasion of Time - back to Gallifrey
The Key to Time - and entire season devoted to this
Destiny of the Daleks
Logopolis - the last Tom Baker episode
Castrovalva - The first Peter Davison and the Master
Four to Doomsday
Earthshock - Cybermen!
Arc of Infinity
The Five Doctors - (well actually four of them)
Resurrection of the Daleks
The Caves of Androzani - Peter Davison -> Colin Baker
Attack of the Cybermen
Revelation of the Daleks
Remembrance of the Daleks
New series are all good!
Go watch it now!