How can I make a list of installed packages in a certain virtualenv?

Calling pip command inside a virtualenv should list the packages visible/available in the isolated environment. Make sure to use a recent version of virtualenv that uses option --no-site-packages by default. This way the purpose of using virtualenv is to create a python environment without access to packages installed in system python.

Next, make sure you use pip command provided inside the virtualenv (YOUR_ENV/bin/pip). Or just activate the virtualenv (source YOUR_ENV/bin/activate) as a convenient way to call the proper commands for python interpreter or pip

~/Projects$ virtualenv --version
1.9.1

~/Projects$ virtualenv -p /usr/bin/python2.7 demoenv2.7
Running virtualenv with interpreter /usr/bin/python2.7
New python executable in demoenv2.7/bin/python2.7
Also creating executable in demoenv2.7/bin/python
Installing setuptools............................done.
Installing pip...............done.

~/Projects$ cd demoenv2.7/
~/Projects/demoenv2.7$ bin/pip freeze
wsgiref==0.1.2

~/Projects/demoenv2.7$ bin/pip install commandlineapp
Downloading/unpacking commandlineapp
Downloading CommandLineApp-3.0.7.tar.gz (142kB): 142kB downloaded
Running setup.py egg_info for package commandlineapp
Installing collected packages: commandlineapp
Running setup.py install for commandlineapp
Successfully installed commandlineapp
Cleaning up...

~/Projects/demoenv2.7$ bin/pip freeze
CommandLineApp==3.0.7
wsgiref==0.1.2

What's strange in my answer is that package 'wsgiref' is visible inside the virtualenv. Its from my system python. Currently I do not know why, but maybe it is different on your system.

You can list only packages in the virtualenv by pip freeze --local or pip list --local. This option works irrespective of whether you have global site packages visible in the virtualenv.

Note that restricting the virtualenv to not use global site packages isn't the answer to the problem, because the question is on how to separate the two lists, not how to constrain our workflow to fit limitations of tools.

Credits to @gvalkov's comment here. Cf. also pip issue 85.