The following people have volunteered to help answer questions in their specific areas of expertise. The mailing list is also a general resource for code-related questions.
Alphabetically by last name:
Co-founded the ZFS project at Sun in 2001; designed & implemented many features including snapshot & clones, send and receive, scrub/resilver.
Broad knowledge of ZFS, but most expertise in DMU, DSL, and ZAP.
Spectra Logic & FreeBSD ZFS team member since November 2011, minor role in the OS X port since late 2012.
Primary areas of focus are in DMU & ZVOL, involving I/O performance, bug fixes, & portability.
ZFS on Linux maintainer; implemented the SPL and adapted ZFS for the Linux kernel.
Broad knowledge of Linux kernel internals and ZFS in general.
Member of Spectra Logic's team working on ZFS enhancements, including Optimizing ZFS for Block Storage.
Broad knowledge of FreeBSD internals, FreeBSD's integration of ZFS, and the DMU and vdev layers.
Developer at FreeBSD and FreeNAS projects, working at iXsystems; Worked on FreeBSD's ZFS effort since 2009.
Broad knowledge of FreeBSD internals and FreeBSD's integration with ZFS.
Added ZFS support to U-Boot and initial ZFS on Linux port to ARM.
FreeBSD ZFS team member since 2010; keeping FreeBSD in sync with illumos and fixing small bugs.
Most expertise in porting of illumos changes to FreeBSD.
Implemented zpool feature flags and background destroy of filesystems.
Most expertise in Feature Flags, ztest, and illumos development process.
ZFS developer since 2005; implemented single-copy ARC and many SPA-layer performance enhancements.
Most expertise in SPA (space maps, ZIO pipeline, block allocation, etc), ARC, and corrupt pool recovery.
Work on ZFS on Linux support in Debian as co-maintainer.
The Gentoo Linux ZFS maintainer since January 2012; many miscellaneous improvements to Linux port.
Most expertise in Linux integration and ZFS on Linux development process.
Please add yourself above with a sentence about your experience with ZFS, a sentence about your areas of expertise, and a link to how to contact you (could be via your User: page on this site).