First, for those unfamiliar with the Montessori learning philosophy, here's a brief description from the Montessori International School:
The Montessori approach offers a broad vision of education as an aid to life... Montessori classrooms provide a prepared environment where children are free to respond to their natural drive to work and learn. The children's inherent love of learning is encouraged by giving them opportunities to engage in spontaneous, meaningful activities under the guidance of a trained adult. Through their work, the children develop concentration, motivation, persistence, and discipline. Within this framework of order, the children progress at their own pace and rhythm, according to their individual capabilities, during the crucial years of development.
Montessori materials for younger children are roughly divided into six subject areas: botany, geography, math, language, practical, and sensorial. Not surprisingly (to ourselves, anyway), we were quite drawn to the sensorial materials. There's something viscerally pleasing about these objects that makes us want to purge all other playthings from our house and rebuild our inventory from scratch, Montessori-style.
The cost of Montessori materials used to be prohibitively expensive, but now that they've opened up to the home market, and with discount retailers like Montessori Outlet and Kid Advance on the scene, prices have come down significantly and are now on par with well-made toys.
And like any well-designed, well-imagined object, Montessori materials have a shelf life way beyond the school years. As adults, we can imagine exploring some of these items long after our kids are past preschool age.