As others have already mentioned it completely depends on what country the hotel you are staying at is located.
North America - generally I believe the housekeepers rarely receive tips - especially on one night stays. If I ever have a special request (like extra towels or another coffee filter or some extra sugar etc) and they help me out I always leave at least $3-5 bucks per night.
Caribbean - if you've ever gone to this area of the world on an "all inclusive" or holiday of any kind housekeepers LIVE on tips as their base pay is practically nothing ($50 USD a month - probably even less).
The weird thing about the "all inclusive" vacations is that the tour operator really reminds you that all tips are included in the price of your vacation package and that you should only think about tipping if you have received exceptional service - and even then you don't have to.
Next time you visit these Caribbean areas take a few minutes and talk to the staff (they are almost always extremely pleasant, helpful, hard working people and you can always find at least one person who speaks your language very well) and they will tell you the truth about their working conditions and pay. It's really sad when you think about how much you paid for your vacation and the people that work the hardest to make your stay is fantastic are the ones that get paid the least.
Now with all that being said my girlfriend and I usually tip housekeeping staff in the Caribbean at least $5 USD per day. We know some people who tip a lot more but we feel that's a comfortable figure for ourselves. Even without tipping you will still receive great service and cleaning - but I really think that housekeeping truly appreciates whatever tip they receive and as a thank you they will go that extra mile to make your room and stay that much more comfortable, clean and beautiful.
NOTE ON WHEN AND HOW TO TIP
North America - leave your tip on the bed that needs to be made and if possible leave a thank you note so the staff know's that the cash on the bed is indeed for them and they are not stealing.
Caribbean - very important that you leave your tip on a daily basis - same as above on the bed with a small note in the local language "gracias" or something similar so they understand the cash is for them.
Now, why leave the tip on a daily basis and not all up front or all at the end of your vacation? I know lots of people that either tip up front or tip at the end of their stay and I always try reminding them why this may not be so great of an idea.
1) Leaving your housekeeping staff a tip for the entire week on the first night.
ANSWER: Bad idea. Housekeeping staff at large hotels/resorts do not work 24/7. The staff who cleans your room on the first night of your stay may not be the same person who cleans it on the 3rd, 4th night etc. If you tip all at once on your first day that housekeeper will be thrilled - however that might be his/her last day on their rotation and they will be off for the next 3 days. Now the new housekeeper comes in on the 2nd day and doesn't receive any tips for the rest of your stay :(
2) Tipping the housekeeping staff all at once on your last day.
ANSWER: Again in my opinion bad idea. Same problems of staff scheduling and days off as mentioned above in number one. You also then have to factor in that the housekeeping staff perceives the lack of tips every day as a sign that this guest may not be a tipper. If they are under this impression they will still work hard and clean your room but the person next door who tips every day will definitely be remembered first and they will be the ones who receive the housekeeping staff's "extra attention" all week when it comes time to clean the next room.
Anyways long story short tip your housekeeping staff for excellent service. It doesn't have to be a big tip - I know that they are thrilled to receive anything that recognizes how much effort and work they put into their job.