However, unless you're careful, these search tools can do just as much harm as they can good and can mean your site isn't really showing all it should to visitors. Classic examples are when price or distance are involved and I've got a website that can demonstrate both: autotrader.co.uk.
If it's not immediately obvious from the domain name, Autotrader are a huge UK car sales site. They were a periodical magazine long before the Internet existed (I'm pretty sure I bought my first car from an Autotrader paper edition) and they've developed a pretty useful website now we're in the online age. However, their search tool sucks, and here's why.
I want a Porsche 911. Seriously, I do: If there's anyone from Porsche reading this I have absolutely no problem accepting gifts, I really don't. In the meantime though I'll use Autotrader to search for one. So I enter "Porsche" as the make, "911" as the model and specify a minimum price of £15,000 and a maximum of £35,000. I say I only want to see cars within 30 miles of my postcode. Autotrader pretty quickly tells me that there are 36 cars fitting those criteria. Cool, that's very good. The most expensive, at £34,995 is this one:
This car is almost the same distance away - 16 miles, well within my 30 mile limit - and is just £990 over my £35,000 original budget. Do you think that if I was going to pay £35,000 for a car that I wouldn't be prepared to even look at one that's just £990 more? Especially when you consider that I might well be able to haggle the salesman down under £35k anyway. That second car may have been exactly what I was looking for when none of the others were right yet I'm going to walk away and this guy has lost a sale.
The same thing happens with distance. If I go back to my original £35k and 30 mile search I can see that the farthest car away is on the 30 mile limit. If I change the distance to 40 miles, I can now see there's another car at just 33 miles and two more at 35. Again, if I'm driving 30 miles to spend £35,000 on a car, I can probably consider driving another 3, or 5 miles.
OK, you've got to draw the line. If I, as the customer, say I only want cars that are £35k or less and within 30 miles then I might, conceivably, if I was very weird, get annoyed if you showed me cars outside those limits. But they could be presented in a special way; if you search by distance they could be tacked on the end with an introduction line saying "You may be interested in these cars just a bit farther away...". If you search by price it could say "These are just a bit more expensive but you might want to take a look." You could use some rules about how much farther and how much more (or less) expensive you'd go depending upon the original values. It'd be a cinch for a programmer. I know, because I've done exactly this on a couple of sites I've built, it's just some simple math that takes a couple of lines of code or a change to the SQL query.
So, if you've got a search tool on your website go and check it now and see if your customers are losing out because it's being too literal. Then change it.