Im gonna go ahead and recommend the
Steam Compact skin since ive been using it for a while.
Screenshots of it:
Installation instructions
Backup your \Steam\resource\layout\ and \Steam\resource\styles\ folder
Extract the archive directly in \Steam\ and replace those files you're asked to.
Select the Compact skin in Settings Interface and restart Steam.
If Steam's updating files next time you start Steam, you have to repeat Step 2 and 3.
Important information
You're installing this on your own risk! Make sure that you have proper backups in case you want to revert to the original skin.
You have to replace Steam's original files in Steam\resource\layout\ and \styles\ for this skin to apply properly. Any update where Steam says it's updating files will most likely also overwrite my skin files, so you will be forced to re-apply it after every such update. This is due to a Steam issue and it will probably never be fixed.
I strongly advise you to disable DirectWrite font smoothing in Settings Interface when using this skin, since these small font sizes aren't even remotely suitable for DirectWrite.
Download Link
Original Thread (currently unavailable because the forums are getting hammered)
screenshot of my own:
best library view!