All major provider have something in this direction (that is, all three with  
real infrastructure, and everything else are just resellers of these three)

Recommendation: A1, it has (at least) two prepaid data plans, best coverage  
(measured, not subjectively), etc., see below. (I'm not affiliated with them)

Don't use: "3", because since they merged with another company,  
you're lucky if you get a connection at all (I experienced it myself)  
  
**A1**:  
  
**Price**: There is e.g. prepaid <s>3</s> 5 GB, valid for 1 month, for €10  
It "can" be used for phone calls too (not included in the price),  
but for speech there are cheaper things.  

**Where to get it**: Every A1 shop (there are enough, especially in shopping centers), Telekom, and sometimes large post offices. In some cases, an ID is required, but usually not. It only takes some minutes until you'll get the card and an information sheet about PIN code, phone numer, etc., and that's it. Note that you need to pay the first €10 for your card there, they won't give it to you empty.

**Sim/MicroSim**: Just say that you need a MicroSIM when you´re there.  

**Reusing**: After the first month or after the <s>3</s> 5GB, it still works with an per-MB-price (given that you have spare money loaded). If you load €10 again, you´ll get another <s>3</s> 5GB for one month (if you want). It hasn't to be right after the first month.  

To reload, without online banking etc., codes for €10, €20 or €40 can be bought in pretty much every super market (Spar, Billa… just ask the cashier to print a code for A1), post office, gas station, A1 shop, etc.etc. Then, two possibilites:  

 - Call 0800664290 (*german-only* computer voice),  
there you can choose to enter a code or/and do some other stuff.  
 - Independent of the language: Type `*102*xxxxxxxxxxxxx#` where  
the *x* part is the 13-digit code and "call" this. After some seconds,  
you'll get a SMS confirmation.
  
**Expiring**: If you don't load any money for 1 year, the card gets unusable.

**Details** (in German): http://www.a1.net/internet/mobiles-internet-wertkarte