What was it that Neo sold in the beginning of the movie?

We're never told exactly what it is but there is speculation that it was a hack to remove either a boot or a breath analyzer from Choi's car.

Here is an excerpt from the original script draft where Neo is providing a service instead of just selling a disk:

NEO What do you want, Anthony?

ANTHONY I need your help, man. Desperate. They got me, man. The shackles of fascism.

He holds up the red notice that accompanies the Denver boot.

NEO You got the money this time?

He holds up two hundred dollars and Neo opens the door. Anthony's girlfriend, DUJOUR, stops in front of Neo.

DUJOUR You can really get that thing off, right now?

ANTHONY I told you, honey, he may look like just another geek but this here is all we got left standing between Big Brother and the New World Order.

EXT. STREET

A police officer unlocks a yellow metal boot from the wheel of an enormous oldsmobile.

INT. NEO'S APARTMENT

They watch from the window as the cops, silently, robotically, climb into their van.

ANTHONY Look at 'em. Automatons. Don't think about what they're- doing or why. Computer tells 'em what to do and they do it.

FRIEND #l The banality of evil.

He slaps the money in Neo's hand.

ANTHONY Thanks, neighbor.

Something that was related to computers and definitely illegal. We arent sure exactly what it is, but the guy buying it seems very grateful.

The purpose was to set Neo up as this character who is good with computers, doesnt respect authority, and is into computers and conspiracy theories. He would rather sell illegal merchandise to criminals than work at the software company, and he doesnt respect the cops or his boss (and will later end up not respecting the authority of the Agents or the Machines).

I don't think some kind of hacker tool would ever be worth 2000, when so many free tools exist.

It's kind of peculiar for anyone to pay that much for just a piece of software in my opinion. But I guess they couldn't really make the hero a drug dealer or anything.

Maybe the gang is planning a bank robbery or something, and need a specific program that's attuned to the bank's computers.

Perhaps the disc contains blueprints of the bank, etc.

Maybe there's some kind of compromising information that the criminal thug can use to extort or blackmail someone, thereby earning a lot of money, OR an access to somewhere, like a bank, or some other place or thing.

It's possible that it's an algorithm like in "The Office Space" movie, or Superman III, where a computer downloads 'fractions of cents' until they make a huge sum of money.

I mean, when you think about it, -what- data would you pay a huge amount of money for? What sort of data could be that valuable? If you were some kind of headhunter or corporate spy, the possibilities really open up. But that thug didn't really look like he swims in the corporate oceans.

It could possibly contain some kind of credit card fraud code, or code generation tool, or perhaps a way to reprogram some kind of security computer or ATM, or whatnot.

I wish they had explained it better, but 'mystery' is always the easy way out for a lazy writer.

I seem to recall reading somewhere that it erased parking tickets from records. I can't remember if that was officially confirmed, though.