If you play guitar (or any kind of musical instrument)
iTunes and Safari are the ones I use the most. I listen to a heap of podcasts, check the weather and radar to excess, look at all the crap you clowns type.

Guitar Tools - gives you a tuner with every kind of guitar tuning pre-loaded, metronome, chord finder, chord dictionary, scale dictionary
Amplitube - the free version gives you a heap of effects (delay, fuzz, overdrive, distortion, chorus, phaser, octaver, amp modelling, another tuner and single track recording. (You can use the internal microphone or you need an adapter to plug you electric guitar or synth. $36 from ebay)
You can upgrade it with 4-track recording and every kind of Fender pedal and amp modelling.
You can import tracks and slow them down by up to 40%
The only downside is it keeps offering new features for $4.99 a pop. I guess you could get the lot for under $100. That replaces several thousand dollars worth of music technology.