[Xeoncross]

You can use HAProxy or the nginx echo module for this. You don't even need to bother creating an index file to be served.

    location / {
        echo "42";
    }

In fact, I wonder if I could use iptables to return a string...

Oh, even better - There's probably a jQuery plugin for this!

[Syrup-tan]

I'd like to see a Unikernel implementation, if we're going for lower level.

An FPGA with a full-blown TCP/IP stack would be cute, too.