In fact there is one - I don't think it's very well known though.
It's been around no more than about a year or two I think.
hostelzoo - Search and Compare Hostels /
A free hostel price comparison tool
Here are the claims they make on their front page:
Hostelzoo is the world’s largest marketplace for hostels.
We offer a free search and comparison service to our customers allowing them to book the right hostel at the best price.
Every hostel in the world that is available to book online is listed on our site.
We utilise the booking engines Hostelworld, HostelBookers, HostelsClub and Gomio to provide the most comprehensive listing available.
Hostels, Backpackers, Youth Hostels and Budget Accommodation, if they take online bookings, they can be found here.
Disclosure: I'm not involved in this project but a former employer of mine is.
travel.stackexchange.com/users/140/hippietrail
No, not if you need anything near completeness
The other answers list some sites, but those are very incomplete .
For example, the Swedish Youth Hostel association members alone has 2–3 members in Kiruna , one in Luleå , one in Umeå , all can be booked online through webforms or e-mail.
None of the search engines listed in other answers list any of those, and no youth hostel I have ever stayed in is.
My only recommendation is to search specifically through the web, through tourist office websites, or through country-specific websites.
Norway has the extensive Reiselivsdatabasen (includes many categories of accommodation, including one for youth hostels and similar low-budget backpacker style), in Sweden many — but not all
— youth hostels are a member of the Swedish Youth Hostel association , and otherwise, it depends per country.
Perhaps the search engines are particularly incomplete for Sweden and Norway and more complete for other countries or larger cities — I have not made a thorough study.
You might find something through them, but do not conclude from a lack of results that there are no hostels.
travel.stackexchange.com/users/2509/gerrit
Hostelz.com is the metasearch engine for hostels.
It combines information from Hostelworld, Hostelbookers, Hostels.com, Gomio, Hostelsclub, Priceline, and HI Hostels and lets you search for available beds and compare prices from all of the booking sites at once, and also shows you how the hostels are rated in each of the sites.
It also lists hostels that aren't in any of the booking websites, and includes directly contact info for the hostels (which the booking sites also don't offer).
It also has both user reviews and professional reviews (with photos).
The site has been online since 2002.
travel.stackexchange.com/users/24438/traveld
Here is a short list of the most popular sites to book hostels, but I don't know of meta-search websites, as there are not that many search engines: HI Hostels hostels.com hostelworld.com hostelbookers.com
Note that not all hostels are listed on all these sites, usually.
Also note that the last three websites are all owned by Web Reservations International , that might explain why there is no meta-search engine.
travel.stackexchange.com/users/3044/Vince
I actually find that on booking.com , when you do a search, you can choose between: hotels hostels apartments guesthouses b&bs vacation rentals
So as long as you check that, it'll search a variety of hostels as well.
However, personally I still find: hostelbookers.com hostelworld.com hihostels.com are the main three that find anything worth finding, in my experience.
travel.stackexchange.com/users/101/Mark Mayo
Is there a site such as kayak, but for hostels instead? There's so many different hostel booking websites, I don't even know where to start.  If there isn't such a site, what is the recommended course of action on finding and booking the most suitable hostel? (Suitable to mean what's needed for one's own needs.)
Is there a metasearch engine for hostels?
travel.stackexchange.com
<online-resources><hostels>
travel.stackexchange.com/questions/20726
travel.stackexchange.com/users/7735/MarkE
Hostelzoo are Hostelz.com are large search engines for hostels, and booking.com allows you to choose hostels or other accommodation types. Hostelbookers.com, hostelworld.com, hihostels.com and hostels.com are also popular sites. There is a metasearch engine for hostels as well as specialist sites for hostels. In addition, booking.com also has an option for searching for hostels.
There is a metasearch engine for hostels called hostelzoo or hostelz.com. Booking.com also offers an option to search for hostels. There are some specialist hostel sites, including hostelworld.com and hostelbookers.com