SF is most frequently, on this wiki, referring to the source code repository also known as SourceForge. However, there are those on the site whose first thought is that SF refers to San Francisco, and others whose first thought is that it means Science Fiction.
Often used in the phrase file a bug at SF. In this case, go to http://tcl.sourceforge.net/ and follow the appropriate link (bug for Tcl or Tk). This page also has a convenient box where you can type a bug or patch number (as occationally found in comments) and get taken to the relevant report.
The SF bug trackers for projects other than Tcl and Tk, e.g. tcllib, are not available directly from the Tcl project homepage.