= History of an individual file =
Text mode
{{{
git log -p <filename>
}}}

Graphically
{{{
gitk <filename>
}}}

= Find revision by date =
This is kind of obtuse, and I really recommend {{{ git log }}} or {{{ gitk }}}. If you insist:
{{{
git rev-list -n 1 --before="2009-07-27 13:37" master
}}}
This finds one revision (-n 1) before a certain date. You can use this with {{{ git checkout }}} (see below).

= Check out old version of a file =
Find the old revision by using {{{ git log }}} (see above).
{{{
git checkout <revision> <file>
}}}

Reverting to the current version (HEAD):
{{{
git checkout HEAD <file>
}}}