How can I start my own mining pool? Is there some relatively simple (and preferably free/open-source) solution for starting my own mining pool? How close to "turnkey" would such a solution be and with pool operators mostly running at 0% fees and living on transaction fees, is it even worth my time any more?
There is P2Pool and Pushpool being the two options to get your own pool running.

These provide the backend services, the GUI must be built by yourself with any features you may wish to include.

The former is almost unused at this time, although considered to be more resilent to attacks (such as DDOS)

If you were to consider running a pool the requirement of a very high bandwidth server is needed as it consists of using a LOT of traffic (depending where your located this may be VERY difficult to find)

Building a user base may be difficult but with merged mining it may increase profit for running such an enterprise.