Thera are 2 possibilities:

1. SIMPA with Web'n'walk data package
T-Mobile (previously: CroNet) voice prepaid is called SIMPA and the simcard cost is 100 HRK with 100 HRK credit.
When having a SIMPA, you can buy Web'n'walk 10 MB data package for 15 HRK (approx. 2 EUR) and it is valid for 30 days.
If you have never used Web'n'walk before, you can test it buying a 10 MB packet for 1 kuna (promotion valid up to the end of 2008).
To activate, send an SMS with a text "A" to 3636. An SMS with a list of options will be sent to your mobile. Send the number related to Web'n'walk as an SMS to 3636 again. Another SMS will be sent to you and should be replied with "DA".
More at http://www.t-mobile.hr/eng/1/10-10-10-41-00.asp

2. A new offer:SIMPA Internet
Croatian T-Mobile introduced a new start package calledSIMPA Internet, dedicated to data transfer only, voice calls are not possible.
You can buy a SIM-card only for 100 HRK with a 100 MB of data traffic included, or the same SIM-card together with a modem "web'n'walk USB stick" - for 798 HRK.
A top up 100 HRK is 180 days valid, an electronic top up is possible also.
More at: http://www.t-mobile.hr/eng/1/10-10-08-00-00.asp