I've been researching this a bit. I know bitcoinjs-lib has a standalone way of generating transactions and pushing them to arbitrary nodes. However, are sites polling third party api blockchains like blockr.io or are they all running nodes for each and every cryptocurrency they support (which isn't cheap really on a hosted machine) and polling them?
Inquiring minds want to know.
Of course the best is to run your own nod. There is other way to use API to any block explorer (just make own address and use their API to track balance and incoming transactions)
Using e.g. websocket is realy quick (you can made exact copy of this page:
https://blockchain.info/unconfirmed-transactions ) .. just read their API documentation