An ISI carbonator - http://www.amazon.co...um/dp/B00007JXR7/ Much less than the SodaStream. Seems to work fine - we've had it about a year and a half and J makes her stash of carbonated beverages about every couple of weeks.
The thing does get quite cold when you're shaking it up to encourage the CO2 to get in the cold water. The SodaStream may be less uncomfortable to use.
We buy 100-packs of the Leland soda chargers (CO2 cartridges) to use with it. They seem to work fine (we haven't tried other brands).
HTH.
Cheers,
Scott.