Tons of stores offer a discount on one item a day until the item sells out, or end of day.
Here are my favorites:
These first 3 sites are unique... they typically offer 50% off of a gift card to a store or restaurant, either local to your area, or online. (They have listings for several cities.) I've purchased from all three of the following and been very satisfied:
These sites offer a specific item (or items) at a reduced price, and offer a new deal each day:
Woot and Kids Woot
There are way too many for me to take the time to link to them all, but here is a great listing of several sites and it updates it with each day's deal and even updates when the deal is sold out: Deal of the Day Tracker
Several of these sites offer RSS feeds. RSS is a standard way for web sites to publish their news feeds. If a site offers an RSS feed, then you can use any of a number of RSS readers to read those feeds. (Google Reader is free and works well. Many web browsers and email clients also have RSS support built in.) You can tell if the site you're visiting offers an RSS feed by looking for this RSS icon somewhere on or near the address bar. The advantage of using an RSS reader is that you have a single site that tells you if any of your subscribed RSS feeds have new activity.
For example, Deal of the Day Tracker, Groupon, and LivingSocial all offer RSS feeds, so you can subscribe to them to have a single place to check for new deals as well as blogs you're interested in following.