City Hostel, Geneva

The best option is the City Hostel. The hostel is in a central location, just a few minutes' walk from the station. From the station head down rue de Lausanne for 5 minutes and make your first left onto rue du Prieure, which becomes rue Ferrier. It is at number 2.
It really is a very clean, friendly place with many facilities - kitchen, tv room, internet stations, lockers and laundry. The staff is really helpful. I think it has great value for the money. I payed 28 sfr. for a dorm bed. They also have a souvenir shop, where you can buy discounted swiss army knives, swiss watches or chocolates.