I believe that Geth is planning to have a pruning function eventually, but it isn't available yet. If you resync from scratch using `--fast` it should reduce the size to around 11GiB. The .ethash folder is used for mining, and it can be deleted if you are not mining. It will be recreated if it is ever needed.

See also [How to reduce chaindata database size?](http://ethereum.stackexchange.com/questions/1940/how-to-reduce-chaindata-database-size?rq=1)

Update: Geth v1.5.0 includes an option for a light client. This will take your chain data down to under 200MB. Just start geth with the `--light` option. You can then optionally delete the main chain data.