Weatherunderground is a great site. Click here[url]

decide which region you want and when you get to its front page look for the "trip planner". It is a geat tool- you will see all the averages for your stay. ( not all destinations have a trip planner by the way)
It is easy for people to make big mistakes in planning holidays in Thailand as I find there is a lot of misleading information out there. Some of the worst offenders are travel brochures where they can just give one set of statistics for the whole country!
this is a useful guide as well

Sunbear is the one who has good weather info websites. Perhaps he can add to here as well?

The Thai Meteorological Society has a useful site with a link to Climate for Tourism...its at
http://www.tmd.go.th/index_eng.php

One of the maps gives average no of rainy days by month which is quite useful