Hyves is quite likely the biggest online community in The Netherlands now. It has extensive possibilities, more than - say - MySpace yet it's futures are a bit unstable implemented by times.
Adium or Adium X is an Instant Messenger client for Mac OS X. The basis is Pidgin (previously "GAIM"), a program for Linux and Windows. This tutorial works for both. Adium and Pidgin enable you to connect to multiple IM networks, like MSN Messenger, AIM, Yahoo! Messenger, ICQ and... Jabber networks.
Jabber, officially called XMPP, is different from other IM networks. It's not a network, it's a protocol. Like e-mail: there's not one central network, you can have a firstname.lastname@example.org and contact email@example.com because you speak the same language. Google Talk uses Jabber, LiveJournal Talk does it, a dozen independent servers do it and so does Hyves. And, just like with e-mail (unless you're using Hotmail), you can use any e-mail program you like - not just the one provided by your network.
Hyves Chat or "Kwekker" in Dutch (login needed for that link) is Hyves' implementation of Jabber. Your Hyves contacts are automatically in your list and you get updated on events like new friends on your profile. Unfortunately, you can't chat with people on other Jabber servers, but you can use your own Jabber client.
You can use the Hyves Kwekker client (Windows only and yet another program on your computer), the web-based version (needs you to stay on that page in your browser) or your favourite Jabber client: Adium. Download and install from AdiumX.com.
After filling in these settings, you should get connected to Hyves and see your Hyves contacts appear in a new group in your buddy list. You could rename this group to "Hyves". Note that not everybody appearing online knows how to use the Hyves Chat.
Suggestions and questions are welcome here (no login needed though comments will be screened for anonymous comments).