What libraries are available for interfacing with the GPIO?
I'm going to interpret what I think you might be asking more broadly as "How can I control GPIO from userland?"
A very nice way to get started using GPIO is using the
/sys filesytem. You can do it all from the command line.
cd sys/class/gpio echo 0 > export cd gpio0 echo high > direction
More documentation is in linux/Documentation/gpio.txt.
I use Wiring PI and it works really great. I use it with language C and had no problems so far. It is easy to understand and simple to handle.
Pi4J allows you to control the GPIO pins using Java. I'm not sure how it works but there's an explanation at http://pi4j.com/
It is really simple to control the GPIO ports in any language as shown on http://elinux.org, so I believe that you actually do not need a framework for the direct control.
You need to know which additional features you expect of a framework, like
- Remote control via a browser
- A pretty browser UI or maybe a standalone program
- Automatic timebased control (cron jobs)
I ended up writing a framework for myself: Control GPIO ports over HTTP and with cron jobs which perfectly fits my needs. I wanted to have something for home-automation. So I needed cron jobs and a JSON web interface to run a native app on my iPhone. Therefore I did not write a browser UI. I believe it would also be good for an alarm system.
I did not find a (complete) list of frameworks yet. There are some mentioned on the forum of raspberrypi.org.