For Christmas, my parents gave me and my boyfriend a SodaStream Penguin Water Carbonator. We love it, and wish we'd gotten one a long time ago. I love seltzer, but buying bottles of it is both costly and ecologically irresponsible. The SodaStream let's us make as much seltzer as we want in the glass carafes. (My mother chose this model specifically because it has glass carafes, not plastic.)
Williams-Sonoma's website indicates that the CO2 cylinders will each carbonate 60 carafes, but my mother, who has her own SodaStream Penguin, says she has rarely replaced her cartridges. I'll report back on how long the cartridge lasts, but either way, it's a wallet- and earth-friendly device.