Monthly
Mainichi: http://mdn.mainichi.jp/culture/wa/etc/haiku/
Haikuworld: http://www.haikuworld.org
Once-A-Year
Snapshot Press runs a haiku calendar contest with an April 30th deadline: http://www.snapshotpress.co.uk/introduction.htm
Tallahassee Writers' Association "Penumbra" contest with a June 30th deadline: http://www.tallahasseewriters.net/Contests.htm
The Mainichi Daily News runs an annual contest, open for entries from April through August 31st: http://books.mainichi.co.jp/HaikuContest/english.html