From your specific mention of calabrase I would assume you're making an African dish of some sort?
Butternut squash is the closes substitute to those wonderful, sweet pumpkins. I've used it often for that purpose, and most African cookbook writers recommend it.
However, if you really have a thing against pumpkins and squash (btw, horticulturally speaking, there's no such thing as a pumpkin), you can substitute sweet potatoes in many recipes. Sometimes a straight sub works. Other times you have to adjust the sweetening somewhat.
Nan: My understanding is that most canned "pumpkin" is actually hubbard squash, because they grow so large that processing costs are minimized. Libby used to actually identifiy their cans as such, but I notice they've gone back to just saying "pumpkin." Try some straight from the can, though, and you'll notice it isn't as sweet as butternut. | 
Calabaza/Pumpkin replacement? 
