During Spring quarter, I was hooked on an online erector-set type game called Armadillo Run. E-dog saw me playing and asked if I had tried Magic Pen. I hadn't, so he showed me the website, and I think Magic Pen does a better job of allowing for creativity. It can also be run from the internet, instead of having to download a file to play.
I think these games would be great for 4th grade and up in terms of building problem solving skills. Armadillo Run adds budgeting to the equation, as you only have a set amount of money to spend on materials each round, and the over-arching goal is to finish each round while saving as much money as possible.
Here are links to the three games I mentioned above. Magic Pen is playable from any web browser, but Line Rider and Armadillo Run require you to download the demo in order to play.
Magic PenLine RiderArmadillo Run