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While looking for a new ircd to power everywherechat.com, we came across Inspircd.
Compared to our antiquated (10 year old) ircd, Inspicd is damned fluid.
The modular system is extremely easy to work with. There are a lot of existing modules, which implement most features you could possibly want. If you really can't find what you're looking for, it's easy to create your own. All it takes is a little code with an extremely straight forward api, and away you go.
It's fast... I mean really fast... With a minimum of memory, it manages to hold upwards of 400 users on a single server, for us. Not a single hiccup.
Stability doesn't appear to be an issue. We've only been running with it for a couple months, now, but we haven't had any issues of any kind. Others I've spoken with have reported much the same.
Regardless of how great the software is... when it comes down to it, your support base comes from the people who use and develop the product... The folks in Inspircd's IRC channel are helpful, knowledgeable, and always willing to help out.
If you're looking for an IRC server which has all the bells and whistles, doesn't eat your server alive, and doesn't drive you mental while you maintain it... Inspircd is an excellent choice.