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|$, pronounced "$nq$-torial". Famously, it extends to a polynomial function of$q$, and when$q=1$we have$n{1\atop \cdot} = n!$(Oops: I left out the$/B$the first time, thanks Jim and David.) Post Made Community Wiki by Ben Webster 1 As mentioned in a comment,$\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$|GL_n(q)|$, pronounced "$nq$-torial". Famously, it extends to a polynomial function of$q$, and when$q=1$we have$n{1\atop \cdot} = n!\$