There are several sites I check often to see what is selling that day.
Woot.com has a wide variety of items. They have a limited quantity, and the item is no longer for sale when it sells out or a new item is listed the next day. (With the exception of woot-offs, which are a whole 'nother post.) Woot lists new items at midnight central time, or 1 a.m. Eastern time. Check out their sister site shirt.woot.com for eclectic designs, also starting at 1 a.m. my time.
If you log onto woot and see something you like, and the "I want one" button is bouncing, that means less than 10% are left...so hurry up and buy it before it's gone! Shipping is always $5, whether you purchase one item or the max of three. You're only allowed one transaction per day per account per site, so if you want two or three, make sure to select the desired quantity before you check out.
I've purchased everything from camera memory cards to bluetooth headsets to roadside safety kits to screaming flying monkeys from woot.
RetailKiller.com is similar to woot but friendlier to morning people. They list new items at 6 a.m. Eastern time. I don't shop them as much as woot but have purchased from them.
And for the parents, grandparents, etc., a site I need to start checking daily is BabySteals.com. They have different deals every day, starting at 9 a.m. MST...so 11 a.m. for those of us in the Eastern time zone. I usually don't find out about their deals until something I want is sold out...so definitely something to start checking more often!