Spelljammer was a highly unusual setting, appling the fantasy genre to the world beyond the clouds, following assumptions of "fantasy physics" and magic and asking what this ment for the wider universe. The end result was a mixture of the profound and the hilarious and paved the way for interesting crossover adventures as characters were able to travel from one fantasy setting (notably Dragonlance, Greyhawk and Forgotten Realms) to another. We also have Spelljammer to thank for Giant Space Hamsters, one of the more unusual creations of the fantasy genre ;-).