If you use a PC then I strongly recommend you get eSword, a more comprehensive review of eSword has now been written so please go here.
It is free bible study software including various translations (such as ESV, KJV but not NIV due to copyright issues). What is really great is the easy access to commentaries and dictionaries when you are reading any given verse. There is a wide variety of add-ons available (such as the dictionaries mentioned), and also lots of different languages supported. It is also simple to read different translations at the same time (up to 4).
Unfortunately the Mac version, MacSword, is not as easy to use or intuitive, but is still worth a look if you don’t have anything at all.