Again there are no "essential software apps" just apps that you prefer to use over what someone else does. The applications that i consider essential will not be useful to you since you do not do the same things on your PC as i do. So me telling you that FileZilla is essential would be pointless. Same thing with mIRC. You are the one who decides what software is essential. You are the one who decides what software you should use or not use. Not us.
I have a list of over 200 applications that i have tried and used in Vista. I have a list of 50 apps that are a must install for me. But they wont be for you. I already gave 2 examples. Others would be Photoshop CS3, Fireworks CS3, AmpFontViewer, RivaTuner, Camtasia, DVDShrink, DVDFab, Opera, Quicktime Alternative, WinRar...among many many others. So you see we cant answer these types of questions for you. Only you can answer them.