About a year ago I had to install a XMPP server on my company's server. I looked around as to what was available and settled on JiveSoftware's Wildfire Server. It connected to my LDAP server for authentication (a major requirement) and looked quite smart. A year on - I'm stilll really pleased with my choice. A colleague recently phoned me up to ask for advice on installing a jabber server - and came back to me massively impressed with the ease at which he was able to do it.
A year on, and now Wildfire has been renamed Openfire. The philosophy of the company behind it is to release both an "enterprise version" and an "open source" version. In all honesty I think this is a good approach to commercialising open source - as they've managed to give SME's a fantastic server - whilst being able to generate some profit for development off larger enterprises.
Openfire also supports it's own user database - but having a LDAP backend suits me much more. You can even map certain attributes to LDAP fields - so the Jabber Server also becomes another front-end for you contact list and addressbook. It's an added bonus.
There are also lots of plugins for it - and plenty of information should you wish to write your own. They're all administered through a really fantastic web-interface. It's probably one of the easiest web-interfaces that I've used. It has everything there, and has improved massively over the last 12 months.
So, if you're looking for a Jabber server - choose openfire.