Sound Curtain is a very, very simple noise masking app that works on the iPhone when you're wearing a set of mic-enabled headphones. It performs much better than its $0.99 price would imply.
There's three options: White noise, rainfall and harmonic tones. The white noise is decent if you've been using a white noise machine for a while, but the rainfall and harmonic tones can actually relax you in addition to its noise masking properties.
They're no comparison to actual noise-canceling headphones, but for $0.99, they made the ridiculous construction mess outside less of a nuisance—probably even tolerable. [Apple]