If you like this sort of thing (and have a Mac), you'll probably enjoy Dash. It's particularly great to load up on anything you feel you might need before heading towards flaky international internet access for a while.
https://zealdocs.org/ was the one I tried, but the main problem is that it didn't support zip/tar files and Windows (explorer.exe mainly) gets very slow when there are >10 000 files in one path.
second this. devdocs is awesome.
Wow it got really lots of new stuff since I last checked! The offline option sounds cool if you're working on a laptop while commuting etc.