On the Origin of Species was published on this day, in 1859. You can download the 1st edition from Project Gutenberg, in several formats. Librivox has the audiobook.
If you want to know more about the book and its context, I recommend the (always) excellent In Our Time BBC Podcast, still available. For everything Darwin, you can always check the Darwin Online.
I made a search for mobile apps related with Darwin, but none of the apps found caught my eye. Any suggestions?