Geneological Software

For some time I've had an interest in my family tree and family history, but have never really been able to find a good way to actually catalog the little information I have as well as build a family tree. I started using GeneBase, which allows others to join in with you, but they don't have a good solution for adopted siblings. You either have to keep them out of the family tree (genealogy purist would choose this) or include them in the family tree as a natural sibling. I don't want to do the first option and given the unique situation in my family, I can't really do adopted siblings as natural siblings and still have a complete family tree. Does anyone have any suggestions for either (a) good geneological software that I could use or, preferably (b) a good geneological website that would allow relatives to collaborate with me? It must allow for adopted siblings (two sets of parents for a given person) and I'm hoping for free (especially if it allows relatives to collaborate with me). I can use Mac or Windows software.

I considered building the site myself, but realized that I barely have time to work on my geneology, let alone build something as advanced as geneological software.

Thanks for any suggestions.