CalcTastic Scientific Calculator Plus (99c on Play Store) offers conversion, programmer mode, and more aesthetic themes/skins than Android's onboard scientific calculator, and Marton Veges' Algeo Graphing Calculator (free) is a highly functional tool with intuitive graphing, range tables, derivation, integration, and more!
Algeo: https://play.google.com/store/apps/details?id=com.algeo.algeo&hl=en
CalcTastic: https://play.google.com/store/apps/details?id=com.shaytasticsoftware.calctastic&hl=en
What's your favorite calculator or math app?