Fox, CNN, AP, Reuters, SKY, IBN, UPI, just to name a few.
I guess you would consider Huffington a hard news source? I liken it to the Daily Show.
Where do you think Huffington, as an aggregate draws their source material if not from AP, Reuters etc?
They are an aggregator only. The main reason NOT to use them is they don't supply their source links easily. But they are very easy to check.
Looks like they use the local/state news sources as the primary, as did Politico, RCP and others.