Probably every major provider has something in this direction
(that is, I know that two have it, not sure for one,
and everything else are just resellers of these three)
Recommendation: A1 has (at least) two prepaid data plans,
best coverage (measured, not subjectivly), etc., see below.
Don't use: "3", because since they merged with another company,
you're lucky if you get a connection at all (yes, I had it. Had. Now A1.)
A1:
Price: There is e.g. prepaid 3 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 3GB, it still works with an per-MB-price (given that you have spare money loaded). If you load €10 again, you´ll get another 3GB for one month (if you want). It hasn't to be right after the first month.
To reload, 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 either "call" *102*xxxxxxxxxxxxx# where the x part is the 13-digit code (maybe it looks complicated, but it´s independent of any language), or call 0800664290 (computer voide, sadly german only) where you can choose to enter a code or/and do some other stuff.
(Or one could use online banking stuff instead of the code number).
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