ScotteVest
Details: Scottevest has a secret "Sale of the Day" page on which they will on (MOST) days feature 1 item at around 20% off. They dont feature an item EVERY day, but the vast majority of days you will find an item listed at 20% off. The page is at http://www.scottevest.com/v3_store/subindex_new_on_sale.shtml ... make sure to bookmark it!
Kidrobot
Details: Get 20% Off Do It Yourself Toys + Free Shipping on Orders of $150 or More
Design Public
Details: Sign Up & Get 5% Off on Order of $50+.