I think that most of us here use a computer in our daily lives so I was curious which is the application/program that brings you the most enjoyment?
For me it definitely is Winamp coupled with MiniLyrics, a program that automaticly downloads the text of the song playing. I don't actually sing along with the songs (when sober at least) but i really enjoy seeing the lines of text scroll by.
Does bash count? I know it's kind of an elitist answer, but I just love the no-nonsense ability to get things done instantaneously and efficiently. I can check my mail, respond to an IM, start a download, and be back to writing in a text editor before Firefox even loads.
Google Chrome, simply because of how easy it is to search for things... If I want to search youtube, I type "y" and hit tab, and I can search youtube from my browser bar. This saves a page load or two, and is extremely convenient. It tracks every single search engine I've ever used, so that I can use it straight from the browser bar.
Tough question, @lane's answer is great - bash makes some tasks much simpler while being extremely powerful. Other than that, possibly TextMate - everything just works, and works really well - it's exceptionally simple, yet has every feature you could possibly want.