While Large Text File Viewer works great for just looking at a large file (and is free!), if the file is either a delimited or fixed-width file, then you should check out File Query. Not only can it open a file of any size (I have personally opened a 280GB file, but it can go larger), but it lets you query the file as though it was in a database as well, finding out any sort of information you could want from it.
It is not free though, so it is more for people that work with large files a lot, but if you have a one-off problem, you can just use the 30-day trial for free.