lightningtimer.net. I'm fed up of having to dig out or knock up a timer script every time I manage lightning talks, so I've given one a domain name. You can use lightningtimer.net/#90 to set a different start time for the counter.
Update March 25th 2025: I rescued an old copy of this from the Internet Archive and re-published it to tools.simonwillison.net/lightning-timer.
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