In short, miau is an IRC-bouncer/proxy.
Being more verbose, miau is small but quite featured IRC-bouncer - a service a bit like HTTP-proxy, but for IRC-networks.
What to use it for
The biggest difference of miau and HTTP-proxy (forgetting the fact they talk all different protocols) is that when IRC-client disconnects from bouncer, the connection to the server isn't necessarily lost. This way your nick can stay online and cannot be taken by the others. In case bouncer loses the nick because of netsplit or other disruption, it can try to get it back automatically.
Some people may also like the fact that when using bouncer, your hostname appearing in IRC is the one of the machine the bouncer is running at, not the one you are IRCing from.
I should rewrite this thing...
miau was written on GNU/Linux (i386), but it should compile on most UNIX-platforms.