If you've found that your MacBook Air's SSD is just too darn small, the folks over at Other World Computing have a kit to help you replace it with a larger one -- and then turn the old one into a super-portable external drive.
OWC has always been great at making MacBook upgrade kits, and this one is no different. It comes with all the instructions and tools you need to upgrade the SSD in your MacBook Air with a larger one, from 120GB all the way up to 480GB. They come in two speeds as well, utilising SATA 3GB/s and SATA 6GB/s (which will give you speeds of about 275MB/s and 500MB/s, respectively).
The really cool part is that it also comes with an "Envoy" enclosure, which turns your old SSD into a small, easy-to-carry-around external drive. The external connects via USB 3.0 too, so if you have a USB 3.0-capable computer (like the brand new MacBook Airs), it will be pretty fast.
It's annoying that OWC doesn't offer a Thunderbolt option, but the cost would probably be prohibitive -- the portability is really what's cool about the resulting external drive. Alternatively, you can buy the Envoy separately for about $US47. Hit the link to check out the kit.