Grab it from mono-project.com.
New features include performance and memory optimizations, compiler support for C# 3.0 constructs, Linq to Objects and Linq to XML. Windows.Forms have been improved (over 100 bugs fixed) and support has been added for the Design-Time framework (System.Design) and the Dynamic Language Runtime. On another note, the latest Mono debugger finally adds support C# 2.0 (you'll need to checkout from SVN:
svn co svn://anonsvn.mono-project.com/source/trunk/debugger).
As you can see, this is an exciting release. Mono 1.9 is now considered feature complete and is supposed to be the last version before Mono 2.0.
OpenTK will currently build and run on Mono 1.2.4+, but newer runtimes provide stability and performance enhancements. OpenTK 1.0 will officialy target Mono 2.0.