An xkcd logo I had been using for a while but hadn't uploaded yet; works on both light and dark backgrounds, with the second logo including reflection.

Edit: Added two more logos, this time without the yellow-weirdness. Again, second logo of each includes reflection.

This logo is best on a dark background.

EDIT
Looks a whole lot better.
xkcd is "A webcomic of romance,
sarcasm, math, and language."

"Warning: this comic occasionally contains strong language (which may be unsuitable for children), unusual humor (which may be unsuitable for adults), and advanced mathematics (which may be unsuitable for liberal-arts majors)."

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xkcd - A webcomic of romance, sarcasm, math, and language - By Randall Munroe, white logo with reflection. Updated: Added Transparent logos

xkcd - A webcomic of romance, sarcasm, math and language.
Ingenious humor by Randall Munroe.