At this point? None. It's a silly exercise on not going anywhere.

HURD uses Mach, which is a mid-90s academic microkernel that's not so good in practice. Particularly, it's so slow at IPC (which is critical to performance in a pure microkernel system) that those using it (Darwin/OSX/IOS, HURD) had to compromise and use a hybrid architecture: Running drivers and some other components of the system with kernel privileges; A popular design choice in the 90s (also BeOS, Windows NT) due to the immaturity of microkernels.

In late 90s/early 2000s, L4 happened. It went to great lengths to actually make achieving performance with a pure microkernel architecture possible. Afterwards, there were a lot of followup microkernels implementing the L4 interface, so these days when we say L4 we generally mean the interface, or any microkernel that implements it.

The HURD was watching, and sometime early 2000s they realized that the hybrid architecture was dead and they needed to move on if they wanted to stay relevant. There was a serious attempt at porting HURD to L4. It was already working, but the people behind it became disillusioned with the HURD, after realizing flaws on the architecture. I recommend reading the papers the L4 port people wrote on this. Back then, there were some ten to twenty HURD developers active.

After that, the HURD should have rethought its architecture and moved on with L4. Instead, they didn't continue the L4 effort nor fix the architecture. What they did was abandon it and start an entirely new port to a different microkernel, Coyotos, which didn't bear fruit, either. Throughout all this, the HURD was losing developers as they became disinterested.

Fast forward to 2005, Andrew Tanenbaum and his students released the first version (a mere skeleton, without even virtual memory support) of Minix3, and continued working from there http://wiki.minix3.org/MinixReleases , going a long way and bringing us to its current state. Right now, Minix3 has 20-30 active developers any given day, a few of which are working on it full-time, supported by funds coming from European Union research programs, as the reliability aspect http://www.minix3.org/other/reliability.html to Minix3 has been deemed important.

Meanwhile, the HURD has 0-3 active developers depending on the day, none of which working full time, and with no roadmap or organization whatsoever. Last I've heard, they introduced userspace driver support, which is a step in the right direction, but a bit silly as the real problem (they're still using Mach) is yet to be solved.

Minix3 next version, 3.3.0, is weeks away. Here's some insider info: It breaks ABI to adopt NetBSD types, skyrocketing compatibility with pkgsrc software. The system will for the first time be built dynamic, as mmap() is finally working and the dynamic library support has been adapted to actually do shared libraries using it.

As lack of proper dynamic library support was holding back X (which already ran, but with barely any software for it), I expect we'll have a lot of WMs, DEs and GUI programs from here on, and interest will pick up.

If you ask me about the HURD, I'd say it's not worth continuing. Just take along whatever salvageable ideas and concepts and move on. Working on a system that isn't at a dead end is one suggestion. Escape https://github.com/Nils-TUD/Escape , HelenOS and Minix3 are three such systems. Genode is not exactly a system but it is interesting in its own way, and Plan9Front http://ninetimes.cat-v.org/ is very interesting even though it is not currently a pure microkernel system (it can be made so without breaking anything thanks to the awesome design). All these systems I suggested are Free Software, interesting, promising and actually active.