This forum has links to the sites which explain the hack and to the software itself, for as long as it’s up.
The hack comes down to filling up your anonymous chipcard once, then making a copy and restoring the copy every now and again. This is undetectable, especially during the week. Weekends are apparently trickier.
Step by step instructions with link here
[edit 15/2/11]Kipje has gone and made OV Station, which allows you to check in and out on the card itself, making it totally undetectable. The sites are down, but you can still find the this proggie which is a GUI to only hack the important data, so it works in around 15 seconds. Unfortunately it looks like they’re detecting this hack and the NS (train services) are blocking hacked cards. Busses and trams still work.[/edit]
And here’s some guy who’s shared his practical experience with the Linux variant of the crack.
There’s another crack, where you prewrite the details of your trip at home, so when your card is checked you’re on a valid trip. You don’t have to use the check in/out points though, making the tampering completely invisible. Unfortunately the software for that is not available… yet…
Finally there’s a more involved method, where you skim the card data from nearby cards (hey, it’s NFC!) and then copy that to your own card.