This is actually quite overlooked, nevertheless interesting: If you have a phrase or a term, eg. a product-name on adwords. You can certainly kind of "track" search volume since you just need to look at the nr. of impressions. So...if you use the usual tools like overture or kw-elite and want to know how much searches are going on....the item is probably still new and doesnt show up in their results. But in adwords you check the impressions and get an idea how often people search for this. This is a no-brainer, sure...but kind of interesting to note.
i do this all the time to test actual search volume and not just an keyword discovery or wordtracker's guesses. Keyword discovery has a free tool that works well ... but there is nothing like actual search terms. the trick is to put the ads on accelerate to get all the searches, and then if you're smart you can shoot for a low position with a crappy ad (no bolded terms) and you can test without even getting a click!