If you want to try out the Beauty Rush nail polishes, Victoria's Secret is having free shipping on ANY order (code: FREESHIP1).
BeautyDeals is having 20% off sitewide (code: 6291), expires November 29th, 2009. Nailtek, Nailtiques, Essie, Orly and others.
Del Sol is having half off of everything, including their color changing nail polish!. Fair warning: I have had good CS with this company but a lot of people have not. Expires November 30th, 2009.
Missha is having 40% off sitewide (must register to get this coupon). They have added two lines of nail polish beyond the small $1 flower bottle polishes. Expires November 27th, 2009.
CherryCulture is having 20% off sitewide (code: TKS20). Great resource for LA Girl, LA Color, Milani, NYX, LA Splash, amongst others. Expires November 30th, 2009.
ETA: some promo codes that people have mentioned in the comments:
OverallBeauty is having free shipping. (Woot! BB Coutures!) Expires November 28th, 2009.
Zoya is having a promo where you get six green polishes (Tangy, Envy, Midori, Irene, Suvi & Veruschka ) with every purchase of $20 or more. Code: GREENFRIDAY. Expires November 27th, 2009.