There are a number of forums online that you can look into. the ServerFault family being some of them, but you already know about those. =)
- For Microsoft based help, try Petri.co.il's forums. There is a gaggle of Microsoft MVPs that hang out there, including Mr. Petri himself.
- TechGenix has a number of sites under its umbrella such as isaserver.org, msexchange.org, windowssecurity.com (message boards forward to security-forums.com), windowsnetworking.com (message boards also forward to security-forums.com) and virtualizationadmin.com. Each has forums with varying levels of activity. Windowssecurity.com's forums are the former Security-Forums.com and as such tends to move well beyond Windows security in spite of the name of the domain. Indicentally, Security-Forums.com will forward you to the WindowsSecurity.com forums.
- Some might cringe at what I'm about to suggest, but I think it should be mentioned: Experts-Exchange.com. And yes, you can really get a free account in spite of their filthy lies to the contrary.
- The new Sysadmin-Network.com is budding (Thanks to Matt Simmons for giving me the tip!) and has the potential to be a lot bigger than what it currently is. Why don't you join and help us grow it?
- I've heard some halfway decent reviews of Daniweb.com's community but have never joined it myself.
- Google has come back with some interesting results that I've never heard of before such as SysAdmin TALK Forums and sysadminforum.com. I can't pledge for their usefulness though and so won't provide a link to them.
- Each individual systems vendor will oftentimes have forums and a community that can bolster your knowledge and community support for that specific item. For example, HP has a vast network of forums for both the IT Pro and the consumer (even if th eoverall experience on HP's web sites is approximately as pleasant as brushing your teeth with a bench grinder).
Notice that most of those area really just help forums and not so much a "hangout". That's because, in the end, I believe that you are right in noticing that there is a lack of a really awesome SysAdmin "hangout". I've been pining for one for years, but haven't quite found it. I think sysadmin-network.com is the closest thing so far, but it could use a little meat on its bones. That will come with time, I hope, as its relatively new.
I look forward to seeing what other people can contribute. Of course, if you find anything on your own, let us all know about it. =)