I went to a demonstration from Clarkmorgan corporate training a few weeks ago and the trainer introduced me to a website called slideshare.net. I checked it out recently and realised that it's a huge pool of sample slides. Some of them are rubbish, however, there are a lot that you can get ideas from. There is one I like in particular called 'Thirst'.

Anyway, I recommend the site, and I think the trainer who introduced me to it was Andy Clark. His presentation was very good.

Slideshare is great, but there are quite a few ppt files on there that are advertising now. Also try running an advanced google search for .ppt files as there are lots of universities that offer their files for everyone.