I'm the Security Officer for the FreeBSD operating system... does that count? :-)
EDIT: I should add that this is one of the main reasons I got started in the field of online backups: There were lots of online backup solutions available, but I wasn't impressed with their security -- and due to being FreeBSD Security Officer, I tend to have sensitive information (e.g., about unannounced security vulnerabilities in OpenSSL/BIND/Sendmail/Apache/etc), making it absolutely essential that my backups be secure.
Yes, I actually just contributed my first patches for Rails this weekend (I think 3 or so got committed already) during the bugmash event. I definitely plan on making more contributions too.
I contributed a lot of code to Caudium back in the early 00's even going to the Pike/Caudium conference in Paderborn, Germany in 2003 (which is quite an expensive proposition for someone from New Zealand).
My biggest open source project at the moment is a Pike library which reads in packets from libpcap and builds a table of active TCP and UDP flows, including all the metadata about the flows (and optionally the payload) with various callbacks so that you can easily build analysis, diagnostic, accounting or logging applications.
I wouldn't say I actively contribute. However, I do contribute when I need something done. For example, I need Spree (open source Rails e-commerce) to handle s3 downloads so I created and open sourced the s3 download extension. I needed attachment_fu (Rails attachment plugin) to use the GD library, so I forked and added that, which later got merged back in. I don't regularly spend a ton of time on open source stuff, but if I need it, I do it.
And I'm releasing an open-source image archiving app soon.
I'll admit, I'd love to contribute to a larger open-source project but I feel intimidated at times. Sometimes I'm not sure I have anything to bring to the table for such projects ... considering the talent that already works on them.
I currently work on the Haiku project as a part of Haiku Code Drive. I wish to become a full time contributor once I'm done with the Code Drive.
Also, I might start writing a FOSS app of my own in the future. It's just an itch I want to scratch.
I can contribute to FOSS because I'm still in college. I guess people with jobs or startups might not have enough time for, um, "extra-curricular" activities.
EDIT: I should add that this is one of the main reasons I got started in the field of online backups: There were lots of online backup solutions available, but I wasn't impressed with their security -- and due to being FreeBSD Security Officer, I tend to have sensitive information (e.g., about unannounced security vulnerabilities in OpenSSL/BIND/Sendmail/Apache/etc), making it absolutely essential that my backups be secure.