How can I view the output of `git show` in a diff viewer like meld, kdiff3, etc

You can use git difftool to show a single commit.

Say you want to see the commit with the sha1 abc123:

git difftool abc123~1 abc123

(~1 tells git to move to the previous commit, so abc123~1 is the commit before abc123)

If you use this regularly, you could make a custom git command to make it easier:

1. Create a file called git-showtool somewhere on your $PATH with the following contents:

   git difftool $1~1 $1
   

2. Give that file execute permissions:

   chmod +x ~/path/to/git-showtool
   

3. Use the command git showtool <sha1 or tag or ...>

4. Profit.

Translating sagittarian's suggestion for the less git savvy, add this to your global .gitconfig file which resides in C:\Users[user name] if you are a windows user:

[alias]
    showtool = "!showci () { rev=${1:-HEAD}; git difftool $rev~1 $rev; }; showci $1"

or you can execute the following command in the get bash shell:

git config --global alias.showtool '!showci () { rev=${1:-HEAD}; git difftool $rev~1 $rev; }; showci $1'

which will make the change to the .gitconfig file for you.

Building on georgebrock's response, you can create an alias in .gitconfig, something like this:

showtool = "!showci () { rev=${1:-HEAD}; git difftool $rev~1 $rev; }; showci $1"

Then you can run it with git showtool abc123 (without needing to create a separate shell script for this). If you leave out the revision it will default to HEAD.