I’m in the final testing stages of developing a module that hopefully will save everyone a lot of time.
Once my module is installed, installing other add-ons is basically a one-click process. You just visit the module page (screenshot below) and then either upload a .zip/.php file, or give it the URL to the source (usually from the developer’s website). Hit one button and my module up/downloads, extracts, and searches through the files and determines where each file/folder goes.
For example, if you just upload a file called ext.some_extension.php, it will get moved to your system/extensions folder. If you give it a URL to a .zip file that contains a folder for a module, a pi.someplugin.php, and a lang.someplugin.php, all of the files will get extracted and moved to their proper locations. If it comes across a file it doesn’t understand, it will let you know.
One big thing that I am already planning on doing soon is having a long list of known add-ons so that installing truly will be one-click and not require looking up a source .zip URL.
EDIT:
STATUS: Released!
http://philfreo.com/blog/freo-addon-installer/
I’m posting the official announcement and support thread here:
http://ellislab.com/forums/viewthread/131282/
Please direct further comments/questions there.