Hi when you planning to go ?  me as well i go to Paris 19 20 march, this is good website :www.hostelworld.com 

If you like to meet people, you can also try www.couchsurfing.org, Paris is one of the most active community in the world.

I´m going to Paris in May with my French class and we are going to stay at Hotel de Roubaix,Rue Greneta 6.

We just have to pay 350 chf for 2 nights and this price is also incl.breakfast,train(TGV),tickets to metro /bus...

Hostels in Paris are notorious for being a bit ... unclean, as well as lock out times, curfews, misc charges for membership, linens, ect.   

Depending on your budget (cheap is relative) you can find alot of apartments for rent on Craigslist.com.  People who rent out apartments entirely, or as well as just extra rooms.  If you use good judgement, you can find many affordable and safe deals.  I found a 1 bedroom studio for a girlfriend for 80 Euros per night, which by Parisian hotel standards is quite inexpensive and she said the place was great.  

If you are familiar with Paris and using the RER staying in the suburbs in a hotel can range about 50 euro per night plus the added cost of the metro.. but this isn't a good option if you're a night owl.

If you do decide to take a hostel, use a hostel finder which lets you pick your arrondissement.  And pick based on the type of nightlife you want to save on taxi fares home.  The 6th is my favorite for nightlife 

http://www.airbnb.com/

I found a very nice flat only for 70chf/night

you can look that!

http://www.booking.com/

SYLWIA - we are going after 3 weeks (after your trip).

Thank you all, for a good tips!

http://www.st-christophers.co.uk/paris-hostels Last time I went to Paris I stayed there. Free wifi, close to things. It was for the leweb conference. Took a double room by myself. Quite practical.