As mentioned in a comment, $\lfloor x\rfloor$ is much better notation than $[x]$ for denoting the greatest-integer function. Most especially since it doesn't collide with the $10^6$ other things that $[]$ is used for, e.g. the $0,1$ function Richard Borcherds mentioned.
I very much like, though haven't had much use for, the notation $n{q\atop \cdot}$ for $|GL_n(q)|$, |GL_n(q)/B|$, pronounced "$n$ $q$-torial". Famously, it extends to a polynomial function of $q$, and when $q=1$ we have $n{1\atop \cdot} = n!$
(Oops: I left out the $/B$ the first time, thanks Jim and David.)
