The del.icio.us API is nice for getting your own info, and the RSS feeds are good for getting recent posts from a user, but something missing is an unauthenticated API that gives you all of a user's tags, or their common tags.
Thinking about Dave Winer's comments on the Amazon S3 API. Dave, how about a local app that uses the sample code or WSDL to connect to S3, and provides an XML-RPC interface? You could do this in Python with the built-in SimpleXMLRPCServer module if you liked; it wouldn't be too much code.
Update: I've made a little S3 XML-RPC to REST proxy in Python, if you're interested in doing this sort of thing. It works as described above; you can talk to it with XML-RPC and it'll talk to Amazon via the Python S3 REST library (included).
That's excellent about S3's BitTorrent support, BTW. So you can now throw a file up on Amazon and get them to seed it, for $0.20/gig to upload, $0.15/gig to store it, and $0.20/gig BT traffic? The prices aren't that fantastic compared with dedicated servers ($100/month for 1000 Gb traffic and ~80Gb storage, which you could call $0.09/gig traffic and $0.07/gig storage), or a shared plan, e.g. $10/month for 750 Gb traffic and 20 Gb storage at netfirms (similarly, $0.013/gig traffic and $0.01/gig storage) or Godaddy's $15/month for 1000 Gb traffic and 100 Gb storage ($0.01/gig traffic, $0.007/gig storage).
If you don't care about traffic, just space, Godaddy is the best deal if you can fill the 100 Gb completely. If you just have a little, it seems like Amazon is way cheaper than anything...