gnuCash is available for all main platforms, Linux/Unix, MacOS (FreeBSD anyway), and Windows.
You can download gnuCash free from here.
It does seem to be fairly full featured and somewhat straight forward, but like so many programs it is missing a few key features or things don't work quite right.
It doesn't support importation of CSV files, which is a big minus. And the developers rather than wanting to add support have just decided to link to a page that describes "how to convert CSV to QIF using a shell script". They go on to say how similar CSV and QIF really are, and I don't mind using a shell script, but this is really not practical for 99% of people out there.
At least it should be easier for users of Quicken/Quickbooks to move over and it's great that it's a cross platform application.
I'm not sure if I'm missing something, but I confirmed my QIF file has records going back more than 12 months, yet after importation it appears to only show the current month and nothing before. This is either an oversight on my part, or a huge bug/feature that needs to be fixed.