Are there any tools to periodically check if a web site is available?

"You already mentioned pingdom.com

The good thing with pingdom is that they are the only free service I know of that offers a 1 minute check resolution. Their free service is restricted to one site, if you want more checks you'll either have to pay or participate in GIGRIB. All other services I found are either paid or their they check only every 5 or 10 minutes.

EDIT:
site24x7.com also offers to check every minute for free, but only from california.
pingdom checks from all around the world."

We use Webmetrics and Keynote is another that has been recommended to me, both online services

"This is a bash solution that I wrote a couple years ago to see if the server is up:

#! /bin/bash
# check to see if site is up
# if it is, don't worry
#
# v.1 Warren M Myers - initial stab
# 31 Aug 06
#

# 7 is the return code of `curl` if it cannot load the page
ERRCOD='7'
WHEN=`date +%d%b%y`
REPT=""/var/tmp/$1.$WHEN.txt""
STARS='********************'

# $1 is the command-line arg for the domain
# you could hardcode this, too
curl -I $1 > /var/tmp/curlret.txt

# $? is the exit code of the last-run script
if [ ""$?"" = ""$ERRCOD"" ]; then
# return was unable to connect to host: save ps -aux; mail report
echo $STARS >> $REPT
echo 'curl return results' >> $REPT
echo >> $REPT
cat curlret.txt >> $REPT
echo >> $REPT
echo $STARS >> $REPT
# mail the whole works to myself
mail -s ""failed to connect to $1"" your.user@example.com < $REPT
fi

rm -f /var/tmp/curlret.txt
rm -f $REPT"

"It's self promotion, but I have developed an online tool for this too:

Port Monitor

It also provides a REST API if you want to integrate the checking in your own app or site."

Update Scanner is a firefox add-on that checks websites and allows per site polling interval settings. It's generally meant for checking changes to a page, but being down would be a change I guess. :-)