Significantly less female participation. It is estimated that female participation on open source projects is at about 2%, compared to 10-30% overall in computing or proprietary projects.
For overall female participation, NSF keeps employment statistics for women, which has hovered around 20-30% for the "computer and math scientists" category.
There are a number of reference to the low participation of women in open source projects; one commonly referenced is Free/Libre and Open Source Software: Policy Support (2006), which lists a number of 1.5% - it's a while ago, but I'd be surprised if the number is much different today. To compare, an earlier study (2002) listed 1.1%.
There are other numbers which suggest that tech companies' employees are about 12% of women.
Now per-project numbers vary a lot and I'm not sure how useful individual comparisons will be, but here are some numbers:
- The same source that lists 12% for a group of tech companies shows Mozilla at about 8%. By comparison, Google reports 17% of women in their "tech" category.
- This blog post mentions 5% of Ubuntu Members as women. From the horse's mouth, "Official Ubuntu Membership means recognition of significant and sustained contribution to Ubuntu or the Ubuntu community." There are over 500 of these. By comparison, Microsoft reports 28% female employment.
If you know of better comparisons please edit them in, but again I want to emphasise that individual comparisons are no replacement for the overall statistics, which show significantly less female participation in open source projects.