What is the daily email limit in Google Apps Script?

Note that as of 30th January 2014 the quota is 100 outgoing emails per day:

Check https://docs.google.com/macros/dashboard for the latest values as "All quotas are subject to elimination, reduction or change at any time"…

You can use MailApp.getRemainingDailyQuota() to get an "estimate" of how many more emails to distinct recipients you can send for the day.

I haven't sent any emails today and the function returned 500, so that confirms the daily limit suggested by Joseph.

You may be interested in https://docs.google.com/macros/dashboard Here you can see every quota ;)