I have a pretty calm betta, but he's lived with several types of mollies and does not mind them at all. I have to be a little careful to make sure he is getting the food he needs, as the lyretail dalmation in the tank will literally steal the food from his mouth if he's eating it too slow, but other than that they will swim togeither or ignore eachother. He also tolerated an orange creamsicle lyretail, which strangely seemed to phase him far less than the previously mentioned dalmation despite its bright color.
If you have a bigger tank, I have found that some breeds of loach can also be a really fun coinhabitants. This depends partially on your betta's personality (as it seems most things do), as an agressive betta (especially if they are smaller loaches) might mistake them for worms and try to chase them. The betta would never catch them, loaches are ridiculously fast, but I'd imagine it would be quite stressful for the loach so if your betta is prone to chasing fish in general (or if you already feed him worms) this would not be a good solution. On the other hand they burrow and are largely nocturnal, so if you have a betta that is mostly fine with other fish but can occasionally be tempermental they can be a good solution as they will mostly stay out of his way in general.
(Loaches are like corys though in that they do best in groups, at least 3 and preferably between 5-7. Like corys they CAN be kept on their own, they will most likely just be rather lonely... Sorry people who were looking for non-schooling options.)