BBC writes about small thin client running Linux (Ubuntu distribution).
The cost is less than 100 pounds, but it looks it needs a separate monitor, keyboard and mouse. It's said to be a good solution for developing nations, but in my opinion may be still expensive - and it needs a server (the article says that 2GHz machine with 2GB RAM is enough for 20 clients).
Such solutions are not very popular at this moment, but they're definintely interesting.