SimpylFold
Because of its reliance on significant whitespace rather than explicit block delimiters, properly folding Python code can be tricky. The Python syntax definition that comes bundled with Vim doesn't contain any fold directives at all, and the simplest workaround is to :set foldmethod=indent, which usually ends up folding a lot more than you really want it to.
There's no shortage of Vim plugins for improved Python folding, but most seem to suffer from cobbled-together algorithms with bizarre, intractable bugs in the corner cases. SimpylFold aims to be exactly what its name suggests: simple, correct folding for Python. It's nothing more than it needs to be: it properly folds class and function/method definitions, and leaves your loops and conditional blocks untouched. There's no BS involved: no screwing around with unrelated options (which several of the other plugins do), no choice of algorithms to scratch your head over (because there's only one that's correct); it just works, simply.
Installation
If you're using Pathogen and Git to manage your Vim plugins (highly recommended), you can just
cd ~/.vim
git submodule add https://github.com/tmhedberg/SimpylFold.git bundle/SimpylFold
git submodule init
and you're good to go. Otherwise, clone somewhere else or just grab the tarball, and drop the plugin file into your ~/.vim/ftplugin/python.
Configuration
No configuration is necessary. However, if you want to enable previewing of your folded classes' and functions' docstrings in the fold text, add the following to your .vimrc:
let g:SimpylFold_docstring_preview = 1
Bugs
If you find any bugs, please report them and/or submit pull requests on Github! Simple is nice, but simple and correct is much better.
Happy hacking!