∰ is technically called VOLUME INTEGRAL in the Unicode standard and is located at code point U+2230. That geeky interlude courtesy of this huge table, which is a part of an even larger collection of tables that list every character in Unicode with enough information for you to use them in posts here. Here is the first table.
That ≆ cool. ✐
I can only see rectangles for several of these, I’m using IE.
My suggestion is that you not use such symbols unless you’re sure most people can see them.
≈
Approximation
this post is radioactive
On the other hand, mischievous people could take advantage of this to make amusingly spurious arguments.
“See, 16 ✐ (z) ✐✐✐% less 389 ✐✐ ✐ (p ✐✐✐ q ✐✐ ) ✐ 42! Math proves Macs are better than PCs! See for yourself!”