Are there any servers out there that actually make use of client certificates? I'm trying to write a mod to my network's IRCd to allow SSL certs to be used in place of services passwords, and I have no idea if the problem is with mIRC or my code. I'm assuming it's my code, but I'd like to verify that mIRC actually uses the private key file.

Maximum-IRC.com has both SSL and non-SSL connections.
The default SSL port is +6697
We have multiple IRC addresses, but you can use irc.maximum-irc.com as the server address, and you'll be routed appropriately.

As this is a network/server function, please join #Max-IRC when you join, as that is the network help channel.

I'm not looking for SSL connections. There's a crapton of servers that already support that feature. What I'm looking for are servers with SSL that require client side SSL certificates.

Although I'm an IRCop on that network, I'm honestly not sure.
If you decide to check it out, I suggest that you speak to Ryu, as he's the network owner and would know the details.