I got a free one at first but wasn't over keen on translation, so bought the NIV version which is only just out, was about a fiver I think so worth having, the navigation is also easier on this one, it has a 'finder' option at the top of every page so you can easily get to a book and chapter. I use it a lot now.
The English Standard Version (ESV) is free - I think it is a more literal translation than the NIV so closer to the original but slightly less readable in theory though I find it is fine. I have that and the NIV version that madhairday linked to and use both.
I think it's a good plan to have as many translations as possible (that's what we do in paper format) Have the ESV, the KJV, the NIV (latest edition because they've ironed out a lot of the crap translation) and the GNB (good news bible)