With OpenStack, I go to their web site and I find nothing but a bunch of marketing crap. Cynical me just looks around there and thinks that some companies have got together to make something look open and look as if there might be some open source code and downloads 'somewhere', but there aren't. This is all to try and protect their expensive 'real' products that they know are probably under threat from a truly open source competitor but they just want to muddy the waters.
I'm not going to defend the website organization, however you can click on Software at the top, then Getting Started in the left nav bar and you'll have most of the options for tire kicking and downloading and installation guides on one page.
There is no 'one place' to go and get everything, except maybe github.com/openstack and that's only for the brave who are familiar with 'python setup.py install'. If you build Apache and Linux kernels from source this is for you. Otherwise your best bet to play with it is to run devstack (don't do this if you are afraid of screen) or use either Ubuntu's or Fedora's packages. Both of these work in a VM with >2Gb RAM if you can live with qemu providing the virt layer.