From a developers view:

You connect to them like any regular Gnutella client (ie., your regular v0.4 or soon v0.6 handshake). Once the handshake was OK, you send out a PING. You'll receive about 10-20 pongs which should automatically be stored in your local host cache. You are disconnected (or disconnect yourself) and start doing your thing. Usually, 10-20 is enough to keep your local host cache running without resorting to any of those sites you mentioned.