While gists provide a nice alternative to pastebins, especially when sharing source code, you might at first find them a bit limited.

Sometimes I want to include a pdf or an image, e.g., a plot or a screenshot in a gist.
While this is not possible using the web interface, it can easily be done by operating on the gist directly with git (gists are git repositories):

git clone git@gist.github.com:HASH.git /tmp/mygist
cd /tmp/mygist
cp /path/to/plot.pdf . && git add plot.pdf && git commit -m "Add plot"
git push

Done :)