Crayon Physics Deluxe from Petri Purho on Vimeo.
It looks awesome and like it would eat hours of my life. Using every shred of my self control, I have not downloaded it. (yet) If you do, please let me know how it is.
[Ed. Note: This game does not currently support Macs. The site says that a Mac version is coming out when they are done with the PC version, but that they "aren't making any promises." If you use a Mac, or are sympathetic to those of us that do, please head over the their website and leave a comment. The more people ask for it, the more likely it is to actually be made]
Thanks to Brian for the link.