My son has thousands of pieces of LEGO, but he has them all dumped in a box. Is there any way to figure out what sets they went to if I don't remember the names of them?
First thing to do would be to get together any building instructions you have and attempt to build those sets one at a time noting any missing elements as you go.
Once this is complete and if you still have unsorted pieces, you could use a website such as Bricklink.com to try to identify the sets and then use Peeron.com to source the building instructions for that set.
To begin this process, try to select an odd looking piece that is uncommon in your collection. Use Bricklink to try to identify the piece and then click through to the catalog entry for that piece to see the list of sets that piece is included in. You can follow the links to get an inventory of the selected set and then see how many of the required pieces you have in the unsorted box.
This is not an exact science and will require a fair amount of trial-and-error, not to mention time, but these two websites are two of the best LEGO resources available for such a task.
Before you start:
To help you identify the sets:
Get to know the following reference sites:
If all else fails, post pictures of the less common parts here and we'll see if we can help identify the sets that they're from.
Here's what I did after finding a couple of old LEGO boxes stashed away in my parents' house.
I did all that (took a while, had over 800 pieces to catalog....), figured out which sets are the most probable ones, and ordered the missing parts (usually from BrickLink.com) to complete them.
It was really an excitement to see some 30-year old lego sets being built again :)