siasky.net is a portal. skytransfer.hns.siasky.net is one way to access this service. skytransfer.hns.skyportal.xyz is another way to access this service. Or you could run your own portal (it takes about a weekend to set up and about $50 of siacoin), and access the service from there.
The magic is that any file you upload to one portal is instantly (within about 5 seconds) available on any other portal. The portals themselves aren't running any sort of DHT or federated network - if you upload a file to a portal, then all portals go offline simultaneously, then you spin up a brand new portal, that brand new portal will still be able to see the file you uploaded.