Avocent makes several products in the KVM/IP space. Not all of them are tied to Windows Server authentication. At the low end, they've got a sub-$1000 single port box that works nicely for front-ending existing KVM switches that have on-screen controls.
We've used and tested 4 or 5 products in this "single port" space. Results have been fair, bad and ugly. I would not consider any of them to be acceptable or better.
There are several issues. As someone else noted, these usually push a viewer to you over either Java or Active-X. The little Avocent uses Active-X, so I have to remember to load up IE before accessing it.
Internal authentication is, in my experience, essential. After all, if you're connecting in to deal with the server that's doing your authentication, you're screwed, yes, there are likely expensive ways to avoid that situation.
Serial redirection and terminal servers are an option, but only if all of your servers support that.
VNC isn't an option, unless you like your terminal sessions going over unencrypted pipes or set everything up to tunnel over SSH or VPN.
Solutions that use VNC direct to the target server are insufficient. If you can't talk to the BIOS of a server that's not feeling well, what's the point? Once a server is actually up, SSH into the server gets you all you need, or VNC over SSH if you must do some graphics.
Mouse control: all of the KVM/IP products we've tested have had serious issues with mouse control. With Windows boxes, we generally do our best to get boxes far enough up to use RDP, and switch to that because it's much cleaner. With Linux machines we find this less of an issue as we don't run consoles in graphics mode, thus bypassing the mouse sync issue.
For the original poster, if you want to have the ability to let customers at the console of their server, but not others, you're going to be stuck using expensive equipment, with the ability to handle multiple simultaneous users, or go with servers that have KVM/IP as an on-board option (Intel's is the one I'm personally familiar with. Someone else mentioned Dell has such too).
We made the move to KVM/IP and APC power cycling/control equipment a few years back and have never regretted doing so.