I've got a 5GB partition that I would like to re-join with my main partition. I've heard that Leopard has "on-the-fly" partitioning, but I'm not quite sure what this means. How would I go about doing this?
If it was created in Boot Camp Assistant, use it to remove the Windows partition. It will do it on-the-fly, and not affect the Mac partition.
Otherwise, Disk Utility should be able to remove it, then allow you to resize the Mac partition - also, on-the-fly. Use caution and make sure it is doing what you intend. It is very verbose about whether a particular partition will or will not be erased.
Before doing any changes to the disk partition(s), make sure you have a full backup of the partition(s) you intend to keep.