I just HAD to submit this, it got addicting to me a lot.
Roblox is a free, sign-up-to-play, downloadable game that uses physics. The graphics involve a simlar style to Lego. You can also make your own place that people can visit. Some examples are obstacle courses, factories, taxi places, hangout spots, theme parks, and even MORE. You can join the Builders Club if you have more creativity. The currency is Tx (Tickets), and for club members, Robux.