Simple modular development kits have been somewhat of a unicorn in the safari of hobby electronics offerings. Many have tried, but to date there have been no break away success story yet. B-Squares is a new contender in the running and they are adding solar power to the mix along with an Ipod dock and an attractive form factor.
Previously we had Buglabs take a shot at this genre of product and it appears that they are still around, but it is unclear how long they will stay with $699 to $1,500 (!!!) price tags. It is hard to imagine anyone who wants to tinker with blocks having that kind of cash to burn to do so.
Modular Robotics has their cute little cubelets which shrink everything down and add robotics into the mix with options for drive cubes and range sensing. Also, a far more reasonable price. These look like great educational kits for teachers that want to get young kids excited about engineering.
B-squares with roughly a $25 price tag per square right now hits a nice price point and are being very smart by leveraging arduino compatibility. (You can pre-order on kickstarter and help these become reality) These stand a good chance of being popular at places like thinkgeek.com.