Lightweight service mocks for simpler testing. SMTP, IMAP, HTTP Proxy, SOCKS4 Proxy, SOCKS5 Proxy, and maybe some other shit. VCR makes HTTP testing fast fun.
Despite there being quite a bit of previous art, most services still aren't fun to test. I also like the idea of creating a living, reference model of past, current, and future internet protocol specs.
No idea. You tell me.