I got 3 suggestions so far, all of them good. Thanks!

I think I'm going to check out Rackspace Cloud Files. Though not free, it
looks like it would fit my need. They seem to have a clearer web interface
and API than other services I've looked at already.  Not requiring a
contract gives me some flexibility to use them on a trial basis and I don't
have that many files at present that setting it up is going to be a hassle.
If I do go with RackSpace, I'll report back what I thought of them.

I had previously looked at Amazon CloudFront, and was initially excited
about it, but finally concluded that it was more cumbersome to manage than
I wanted. I haven't ruled it out though. Some sort of web app front end
might swing me back.

CloudFlare's CDN has the great price of free, but the complication to this
one is that you need to use their DNS server for your site(s), effectively
routing all your website's traffic through them. They provide not just CDN
and unlimited free bandwidth, but website acceleration, server scaling, and
security services (even with the free account). However, for my situation,
where the purpose of the files I want to host (at least right now) are to
overlay my library branding and navigation across several domains,
including 4 vendor-hosted services, this just isn't going to work.
Libraries are a messy use case, and the DNS part is not under my direct
control. I can see it being useful for a single site though.

Thanks,
Tom