Photoshop => GIMP (2.8 with single-window mode GUI), or Krita (for raster graphics creation) plus digikam (for photographic manipulation) http://www.digikam.org/drupal/about?q=about/features
CAD => Bricscad http://www.bricsys.com/en_INTL/
tax software => is a problem, admittedly, but I have an accountant who does my tax. For tracking my personal finances, I use Skrooge http://skrooge.org/
MS Office => LibreOffice or more recently, Calligra Office for compatibility with the newer XML formats http://www.calligra-suite.org/
"Important killer apps" are considerably better catered for than games.
For example, a copy of MATLAB for Windows would cost one perhaps $99. I can legally run Cantor with a GNU/Octave backed for free.
There is no lack of excellent desktop software for Linux.