I suspect one reason students test poorly in estimation is because students have ready access to calculators, estimating skills are less important in their daily life. An accurate estimate provides feedback to students on how reasonable their answer is. Most students probably think estimation is just another skill to be tested so we need to put it into the proper context.
I did find some interactive estimation activities online to help students develop their skills in a game format. Glowla’s Estimation Contraption from CyberChase requires intermediate grade students to estimate answers when adding several numbers. Curious George’s Super Bouncy Blast Off is a game for primary grade students. Jan Brett has a Sea Shell Estimation Game on her wonderful web site — always a visual pleasure. (She also has some great movies on how to draw animals designed to be used with children.) For grades 3-12, students can use two estimation methods for measuring a treasure trunk of pearls in How Many Pearls.