Nearly each application needs to have a well-designed GUI. Maybe it’s confusing, but the best application would not be accepted by clients when it has an ugly user interface. So next to colors and structuring it’s probably a good advise to have some well-designed icons in your application. When you search around you’ll find thousands of ‘free’ icons but in most cases they are outdated (WIndows 3.1 style) or only free for personal use, so you could not use them in your applications. I’ve found iconshow.de which offers currently more than 14.000 (!) icons. Of course this is not freeware but currently it’s 139,- € for all 14000 icons (prices might increase when new icons are added). Once you bought it you’ll get a lifetime update license so you will get all new icons for free! For me it was a good buy, definitely.