I tried OTS one year a few years back, and if I recall correctly, I had
problems figuring out how it arrived at the numbers it did. I'm sure I
could have eventually done it, but it wasn't making it any easier to do my
taxes, so I dropped it. However, that was a few years ago, and is just one
I discovered it shortly after giving up on OTS. The guy doing it has been
at it every year since 1996. It's basically a tricked out Excel
spreadsheet with some macro voodoo going on. Seems to work well in
Gnumeric and presumably OpenOffice (though I haven't checked this year's
yet). It does some surprisingly sophisticated stuff under the hood; it's
pretty amazing how little one needs to enter to find a bunch of cells
already populated with the correct info.
It's not explicitly open source, but it's free donationware, and seems to
be more or less in the spirit of open source.