Adobe Photoshop and Photoshop Elements
Dreamweaver
PHPEdit
Poser
WS_FTP Pro
Absolut Telnet

These are the main reasons my last conversion to Linux failed - I simply could not find anything comparable on the Linux platform so was constantly switching between 2 computers or dual-booting (both of which became very old very fast).

Some other major apps that I use a lot but did not cause problems when switching to Linux - Thunderbird, Office