Not a redlining control, but: Aspell.Net is a Free and Open Source .Net spell checking component. Based on the GNU Aspell project, Aspell.Net is one of the most powerful multi-lingual spelling engines available. The API is written in C# and communicates through a C++ wrapper around the Win32 Port of Aspell's C API.
Source repository at sourceforge, checked February 2010 (Tahnks, @magnifico).
May 2012, source no longer accessible... sorry.