Rock-a-Stack
Fisher Price/Mattel, 6 months +

This is a great toy.
Through Alex's first Christmas we ended up with two of these toys, and they are fantastic. The rings are the right size and weight for small hands to learn about grabbing, and they slide along hardwood floors great, so we have been sliding them into each other (in the curling style). Alex has figured out how to disassemble the rocker base, which also provides more entertainment.
In the natural course of things, two of the rings made their way upstairs and are now semi-permanent bath toys, where they get played with nightly.