Yahoo used to carry quite a few tracks up until a couple weeks ago. Now they've re-done it and you have to join either Ubet or one of the off-shores. It's too bad; the Yahoo video was solid.
You can still go to some of the individual tracks web sites and get video. But I believe all the CD-owned tracks (CD, AP, Crc and Hol are all running now) make you join Race Replays.com. Aqueduct and Keeneland provided video from Race Replays.com free but I believe Belmont makes you pay.