Now, trying to think of simple programs that I use that I really like for that reason...
ALSong, ALPlayer and ALZip. (Disclaimer - I used to work for the company.) They're simply excellent.
Skype. Very simple & easy to use.
But I don't really have many programs that are simple... Most are pretty complex.
Once I sort of start looking at other programs that I really like and use a lot, probably Screenshot Captor is next. It's fairly simple, but not all that simple. It's got a lot in it though, so I don't know if it really counts as simple. But for other software, I've tended in the past towards things not complex.
There are some developer tools that I'd consider simple, but really, nah. They're not simple in the normal sense of the word.