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Pokerstove is free and very usefull. But it's pretty damn hard to enter in the information during a hand fast enough, especially if you're multitabling.
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I usually use
www.pokerhand.org while playing. By quickly submitting the hand there you can click "flop" or "turn" to see the equity vs. villains hand. Vs. his range you need pokerstove though. On the other hand you PokerStove isn't that slow to use once you get used to entering hands and can be done while playing, although a detailed and more mindful review afterwards often is a better idea.