Corkboard Connections: Fun Countdown Timers for the Classroom. Not an app, but online timers that may be used on a Smartboard.
7 apple themed math ideas for kids. This article includes free numbered apple circles and many ideas to us them. Great for preschool, pre-k, kindergarten, and early childhood classroom math centers.
Gardening Playdough Mat Printable (The Crafty Crow)