This activity was once a very popular pastime of the poor and the nerdy.

Some who did it became famous,

while some became infamous:

In fact, it became quite a significant cultural phenomenon, managing to get references in Hollywood:

What activity is this that became a “classic” pastime? Who’re all these guys? (The pictures are full of clues )

Partially cracked by: Biki , srichand , Goutham , Mighty Anon , jayanth , shivani , Poornima , and Gammafunction. I’m feeling generous again.

The complete answer I was looking for:

1. Steve Jobs and Steve Wozniak. They spent their formative years making Blue Boxes (see link).
2. Kevin Mitnick - The infamous “homeless hacker” started his hacking career through Phreaking.
3. Scene from the movie “Sneakers” where the character called Whistler is seen holding a Cap’n crunch box. Toy whistles found in Cap’n crunch boxes were used to generate the 2600 Hz tone (see question title) to phreak Ma bell’s phone networks. No one seemed to identify the movie.

The connect: Phreaking.

Biki, srichand, Goutham, Mighty Anon, ganesh, jayanth, Tathagata Chatterjee, shivani, Poornima, Gammafunction and Shashank