The NBI is a PC only database available on CD (see the website for system requirements). It is described as a complement to the IGI in that it does for burials what the IGI does for baptisms and marriages.
It is only an index, so don’t expect to find full details, and like any indexing project it is liable to errors, so always check the original.
The third edition contains 18.4 million entries, covering most of England and Wales. Some counties are seriously unrepresented, for example there are only 4,000 entries for Devon and more importantly for me, only 12,000 for Hampshire. I am pleased to say Sussex is now well represented. The FFHS website includes a useful breakdown of the coverage of the NBI, down to which parishes and date ranges are included.
The NBI retails at £30 for the full version and £15 for an upgrade from the second edition (if you return the first CD of the second edition). It is available through the FFHS and other genealogy suppliers. It is also listed on Amazon.co.uk although as I write this it is showing as out of stock.
Findmypast.co.uk includes records from an earlier edition of the NBI, but there is no indication whether this will be updated with the records from the new third edition.