Source Code
OpenTK relies Subversion for its version control. The source code is hosted on Sourceforge and is freely available under the MIT/X11 License.
To checkout the latest revision, install the Subversion client (link) and use the following command:
svn co https://opentk.svn.sourceforge.net/svnroot/opentk/trunk opentk
You may also browse the source code online at http://svn.opentk.com.


Comments
Re: SVN
A popular subversion client for Windows is TortoiseSVN: http://tortoisesvn.tigris.org/
Re: SVN
... with Tortoise, you can simply use the explorer context menu entry "SVN Checkout". Enter as "URL of repository" https://opentk.svn.sourceforge.net/svnroot/opentk/trunk.
Re: Source Code
and if you use visual studio http://ankhsvn.open.collab.net/ is nice too... i use both
Re: Source Code
If your preferred source control is git, you can use the builtin svn bridge to get a copy of the source locally using the command:
git.exe svn clone "https://opentk.svn.sourceforge.net/svnroot/opentk" opentk -T trunk -b branches -t tagsTortoiseGit will also let you get a copy by doing a "Clone", setting the URL to "https://opentk.svn.sourceforge.net/svnroot/opentk", and checking "From SVN Repository".