Module Manager enables you to browse the modules directory on modules.processwire.com, download, install or update them.
When installed you'll have a new admin page under "Setup", feel free to move it to wherever you like. On first load it will download and cache a json file from where it will look for modules already installed, new versions, or modules not yet installed or not downloaded and provide actions according to its state.
There's a refresh button to look for new modules already put in modules directory and refresh the cache file with the remote list of modules.
If you download a module it will create a temp zip file in the assets folder, extracts it, creates a new folder in /site/modules/ using the class name, and move the files in there. After it's done it deletes the temp files. It will show a message and show an install button, which you can use to install it, but don't have to.
If you update a module it will do the same process as above, and just replace the current files with the new ones.
This version is still alpha and in testing. Feel free to try it out own your own risk. Current version uses still file_get_contents" php method to retrieve the json feed. This requires the server to allow it. Download of the module zip is done using cURL. Also on my local install I had to adjust the /site/modules/ directory to have write permission using php.
Currently it does hide modules for Language Packs, and Admin Themes, since those would require a complete different install routine. Although it should be simple for admin themes to get it to work, but I first have to make sure regular modules work.