Last-Entry is a project mainly written in ..., it's free.
Plugin to find the last entry published in one or more weblogs
Get the last entry date from one or more weblogs.
Usage:
{exp:last_entry weblogs="default_site|my_second_weblog" format="%Y-%m-%d"}
If you'd like to find future entries as well add "allow_future_entries='yes'" to the syntax above.