Google Calendar API:
http://www.google.com/calendar/event?
action=TEMPLATE
&text=BANDNAME @ VENUE
&dates=STARTDATE/ENDDATE
&location=VENUE
&details=FULL BAND LIST
&trp=false
&sprop=LINK TO LMS GIG PAGE
&sprop=name:LEEDS MUSIC SCENE

Yahoo Calendar API:
http://calendar.yahoo.com/?
v=60
&DUR=0400
&TITLE=BAND @ VENUE
&ST=STARTTIME
&in_loc=VENUE
&DESC=FULL BAND LIST
&URL=LINK TO LMS GIG PAGE

iCal Calendar API:
BEGIN:VCALENDAR
PRODID:-//LEEDS MUSIC SCENE//EN
VERSION:2.0
BEGIN:VEVENT
SUMMARY:BAND @ VENUE
PRIORITY:0
CATEGORIES:GIG
CLASS:PUBLIC
DTSTART:STARTTIME
DTEND:ENDTIME
URL:LINK TO LMS GIG PAGE
DESCRIPTION:FULL BAND LIST
LOCATION:VENUE
END:VEVENT
END:VCALENDAR


http://davelms.co.uk/2006/09/04/add-to-calendar/