A previous Answers.OnStartups question asked about the best startup pitches people had seen. Worth taking a look through.
Some other resources for startup pitches are:
And conferences like DEMO and TechCrunch 40 / 50 usually have pitches that are a lot more prepared for (but not necessarily better than) the pitches on sites where anyone can upload.
That's probably enough video to keep you busy for years. Don't spend all your time watching other people's presentations. Read up at http://slideology.com
, make a first version of your pitch, and put it up at the first three sites; improve it by getting feedback.