For nice low crowds, January and early February but the weather can still be quite cool, even cold and there are many Rehabs and closures that take place at that time of the year.
May is quite nice, weather has warmed up and crowd levels are still pretty low, most schools in the US don't get out til June, that's a food thing :)
September is a good time for Low Crowds and Disney Resorts have, the past several years, offered Free Dining, a very nice deal
October is lovely for low crowds and good weather, still quite warm. The Hallowwen Festivities begin and the Epcot Food and Wine Fest is an Event worth exploring
November and first two weeks of December is great for Low crowds and many of the Christmas Events start taking place. It's a very festive time without the Christmas crowds. Weather can be a bit iffy tho. Would avoid Thanksgiving during that time frame however, that week can be a tad crowded