Getting OneTime

I recommend OneTime 1.81 right now. When development on 2.0-beta3 is complete, this page will be updated.

Download the full package as a .tar.gz file: onetime-1.81.tar.gz (or onetime-2.0-beta3.tar.gz).
(This is the recommended way for non-Windows.)
Download the full package as a zip file: onetime-1.81.zip (or onetime-2.0-beta3.zip).
(This is the recommended way for Windows.)
You could just download the standalone 1.81 script (or 2.0-beta3 script) directly, if you don't want the README, the test suite, or other incidental files.
OneTime 1.x is packaged in Debian GNU/Linux now. If you run the "testing" distribution of Debian or higher, do 'sudo apt-get update && sudo apt-get install onetime' (or the equivalent in your preferred package management interface).
Note that as of January 2014, the version of OneTime packaged in Debian is somewhat behind the upstream release. We'll try to get Debian updated soon after OneTime 2.0 comes out.
Gitorious: gitorious.org/onetime
GitHub: github.com/kfogel/OneTime