Hi, I've heard good things about the Benjamin Moore one and the Sherwin Williams one. Pantone also has a palette thing, but though I downloaded it, I never really mastered it!

One thing that is VERY important to remember with colors online is to do test patches in your room. Let two coats on the test patch dry before judging. They can turn out totally differently than they look online due to lighting, what's around them (trim colors, floor, furnishings, etc.). I know my brother re-painted his entire house twice because what he had thought looked perfect on a cloudy day was way too garish in full sun!