One of my current projects is compiling a kiyose, or collection of kigo. Currently, it consists of over 500 entries. When I have completed the collection, I will make it available on this web page, either as a searchable database or a small application.
Until then, I recommend referring to the appendix of William Higginson's The Haiku Handbook or to Haiku World : An International Poetry Almanac. In the former, he has included a comprehensive list of kigo. The latter is a saijiki with examples of haiku from around the world.