Start an HTTP server with ruby -run
Ruby 2.0 has a library called "un", which you can require with the require flag "-r" (-run). The library contains a few helpers for OS-agnostic operations such as creating directories, touching files, etc...
One of the helpers, httpd, runs an HTTP server with the passed directory name. To serve the current directory:
ruby -run -e httpd . -p 5000
via @snatchev
Update: @StephenBallNC pointed me to the Big list of http static server one-liners. Thanks!