Now that I'm involved with Oracle's Cancer Cloud initiative, I'm learning a whole new data domain -- Cancer Research. There's a bunch of publicly available cancer research data that I didn't know existed, and it's really interesting to poke through it, even though I'm still learning what it means.
The government has some very large data repositories, and the most interesting is the NIH's Cancer Portal site. There's a bunch of data there, including the often use TCGA data. This is also viewable in the Exploration tab.