A while back I was working on a mod_rewrite compatible proxy server written in C. The reason for this is that my current webhost uses Apache2 in front ofa number of thttpd processes, and I'd like to remove apache and use something smaller/faster/neater.
Dividing things up I'm running about ten domains, and only around half of them use mod_rewrite rules - small enough perhaps to port the rules, large enough to make it annoying.
Time will tell whether this is a waste of time or not, but I'm confident I could listen upon *:80 and route requests to localhost:N for a few domains.
As a trivial toy I wrote a simple transforming proxy last night which performed simple rewrites as it passed traffic about.
Another fun, and possibly useful, thing I put together was a node.js port of the simple httpstat.us website. That server would have been easy to write in any number of languages, but in node it was almost too easy.
ObQuote: "I just need you to stop being nice to me unless you're gonna marry me. " - He's just not that into you.