Neave Planetarium. Cool flash site where you can see the sky at any point of day/night, at any date/time, from any location. Shows constellations, star names and magnitudes. (Nitpick: Doesn't show planets, bummer.)

(Via ElsaElsa. Yah, that's an astrology blog, deal with it. ;-)

On a related note, I still use the magnificent Skyglobe for these purposes... it's an early-90s DOS program, but it still rocks. :-)