Whenever looking for an hostel I mostly used hostelworld. It used to contain most hostels and was a good resouce. Lately, I am starting to get the feeling that I see a lot of hostels, on the spot, that I didn't find on hostelworld. I heard from a friend working on an hostel, some time ago, that in some cities they were not accepting new units. Is there a more compreensive resource listing hostels on the Internet?
There is no complete resource, for good reason: commercial.
Hostelworld has agreements with many hostels to show them (possibly for money). So that's one collection.
Hostelbookers is similar, and has a slightly different set in certain cities as they have agreements that HW doesn't.
Then there's HIHostels but that obviously only indexes the Hostelling International Hostels.
Google also has a few - if you search 'hostel' on google maps in the city you're in, it'll show some. Again, some, not all.
Long story short - not everyone knows about every hostel. There are some that work on word of mouth. Others are chains. Others only show up in the Lonely Planet books. There's no one resource, but in my experience, I go to hostelbookers first because I like them more, then hostelworld as it seems to have more. Unless I'm specifically after an HI hostel, then I go for HiHostels.
In Central Asia (the stans) they only showed one at the time in Kazakhstan and it was more of a guest house, and all the ones I found in Uzbekistan were word of mouth from other travellers. Go figure.