The blockchain is eating into my macs disk space. Will there be a way in the future to compress it or redirect to an external hard drive?
There is ongoing research into light clients, these are clients that do not require an entire copy of the block chain to function.
I believe you conflated the terms 'memory' and 'disk space' in your question. A light client would require less disk space to function.
More details can be found on the ethereum wiki.
Interestingly it was just a week ago that the first transaction was sent from a light client.