Possibly hundreds or thousands of pieces from a variety of sets procured over the years. The sets get made a few times and then the pieces get thrown into a bin, jumbled with those from other sets. The instructions get stepped on, dog-eared, maybe even recycled. And then your kid comes around one day and says, "I wish I could build [Lego set name here] but I don't have the instructions anymore."
What do you do? You head over to Brickset.
Think of Brickset like your own personal Lego inventory. Brickset's content is sourced from the Lego site, but organized, catalogued and searchable in a way that makes it incredibly useful. You can search for sets by product line, by name or by set number and retrieve a plethora of information : each product page contains product images, downloadable instructions, and parts lists. That's way more than is available on the Lego site and contains everything you need to build a set. Except, of course, you'll still need to sort through all your Lego to find all the right pieces. :)
But what if you're just doing some research on a set that you think you might want to buy? Brickset also includes product reviews, where to buy and even videos. It works decently on tablet, but there's no mobile variant, so it might be a bit slow or small if you're standing in a store trying to view it on your smartphone. Regardless, it's still very handy in a pinch.
The site is also a social network: you can create a profile and inventory all of the sets you own, making it that much easier to locate instructions when you need them. We haven't gone that far, but we have used it many times when the Dude wanted to look up instructions for sets his friends have, and try to determine if we have the right pieces to build it (or hack it into existence). He'll just grab my tablet, head to the playroom and see what he can put together. It's been our saviour in "I don't know what to build" moments and rainy days.
Of course, they have so much indexed that it can invariably lead to purchasing requests, but we like to stifle those with reminders of what's already in the playroom:
|10,000 pieces of Lego (Note for scale: that's a 15" laptop, not a netbook :)|