In modern mathematics, the use of QED to indicate the end of a proof has been replaced by a square known as a 'tombstone'. Less commonly, it is sometimes referred to as a 'Halmos', in honour of Paul Halmos, the mathematician who popularised its use (Halmos is also credited with the introduction of the abbreviation 'iff').

It seems to be a matter of preference as to whether the square is filled or not- as a UK undergraduate, the empty square was rarely encountered, but that seems to have been Halmos' original choice. Unicode merely specifies an end of proof symbol U+220E, which renders as empty in Internet Explorer, but filled in Firefox: your browser presents it (if at all) as ∎. Default behaviour in AMS LaTeX is an empty square, which is automatically placed at the end of theorem proofs; for those who prefer a solid square, include the following code in the preamble:

\renewcommand{\qed}{\nobreak \ifvmode \relax \else
    \ifdim\lastskip<1.5em \hskip-\lastskip
    \hskip1.5em plus0em minus0.5em \fi \nobreak
    \vrule height0.4em width0.4em depth0.25em\fi}

Perhaps the strangest variation I encountered was by one of my Russian lecturers, who would use a circle instead of a square- indicating the start of a proof with an empty circle, and the end with a solid disc. He applied the same procedure to examples, but with triangles instead of circles. I suspect this was a personal quirk rather than an established tradition, however.