I would like to compose some experimental music. For this task, I would like to use any "easy-to-use" software (like garageband, or even a tracker like renoise) but I want to compose my music by specifying frequencies (like 440Hz or 237.25Hz) and not notes (like A4, B5#). Is there any software that I can use to do that and get some .wav or .mp3 easily ?
You can use any synth that will let you work with frequencies instead of notes, like Absynth. You can assign specific frequencies to each oscillator, and/or assign specific frequencies to each MIDI note/key. You can use it with the host of your choice (DAW, tracker, external recorder, whatever).
For more flexibility you might want to look into audio synthesis environments like Reaktor, Pure Data, SuperCollider, and MAX. You'll have more control over all the details, at the cost of some more complexity.