Mathematical art

Carved wooden möbius strips, courtesy of this post on mathart.

Also, beautiful mathematical metal sculptures.

I love seeing the tangible beauty in math, especially when I'm studying not very interesting math for exams. (Introductory differential equations, right now. None of the theory, no overarching elegant concepts that fit together and make sense, just a bunch of rules to learn. Introductory applied math courses aren't much fun.)