I really enjoy YouTube. It's a great place to find tv shows, music videos, movie trailers or just a laugh-out loud snippet. It is very easy to use whether you are simply browsing videos or uploading your own. I personally use it both ways and often post concert videos when I get a chance.
My only complaint about YouTube is that there is just so much out there. Everyone names there videos so similarly and often times inappropriately that you don't always get the best results. For example, try looking for an official movie trailer and you'll find all kinds of videos not just the studio's version.
As far as applying YouTube to library services, it would be great to have video of some of our fantastic festivals. It's a lot more interactive and attractive to customers, I think. Also, there is a new trend of book trailers (similar to movie trailers) and I've always wanted to create a teen program in which the teens write and act out a trailer for their favorite book. I've just never come up with a good arrangement for this. It's also a great place to find author interviews to use with book clubs.
Check out this video about my favorite character...Babymouse!