Theme Signal is a directory of commercial WordPress themes. It was created to help individuals find exactly the theme they’re looking for. Right now, there are only a few themes in the directory so its usefulness is limited. With time (and more themes), it should become quite useful for those looking for a particular type of commercially supported WordPress theme.
For full disclosure, it should be noted that Theme Signal uses affiliate links for outbound links leading to theme author sites. This is done in an attempt to make the site sustainable and allow its continued development.