WARNING: GEEK ENTRY
I'm in a Biblical Studies program, so I have to read the Bible a lot. Often, when I need to copy and paste portions of the Bible into a paper, or just for study, I go to Biblegateway.com. Biblegateway provides a large number of translations, but does not include the RSV or the NRSV, versions which I prefer to the NIV/TNIV. So, I went shopping around and stumbled on this amazing Bible website. Strangely, I can get the RSV, and NRSV at The Unbound Bible, but I cannot get the NIV. Maybe there's some Coke vs. Pepsi thing going on with with two translations, whatever.
The reason this website is so great, is that I am able to examine the text in four different versions all at the same time. And I don't have to rely on translations either. There I can find the BHS with and without vowels, the Septuagint with and without accents, as well as with and without verb parsings. They have the Leningrad Codex, the Aleppo codex, and the Peshita, along with a myriad of other translations. Though it may be missing the NIV, the versions that it does have vastly outweigh that omission.
In short, if you're leading a Bible study and want to use the NIV because of its simplicity, or if you're just looking to examine a bunch of different English translations together, then go with Biblegateway.com. (But keep in mind, you'll be missing out on one of the best English translations to date, the NRSV.) If you want the scholarly tools, then definitely check out The Unbound Bible. (Oh, and the best thing about The Unbound Bible, is that the Hebrew copies and pastes right into Mellel, with perfect pointing. You just have to change the font to the SBL Hebrew font and it works great... now if only Times New Roman would learn how to point Hebrew...)