As a contributor to several open source projects myself, I know how rarely you hear "thanks". So:
Thanks very, very much for all you've done on md/mdadm. It's a brilliantly reliable, solid and easy to use system that I strongly prefer to fakeraid/hostraid setups and almost always favor over true hardware RAID as well. It's a sysadmin's dream.
About the only time I ever use anything else is when I'm not using a Linux server or I need write-back caching, in which case hardware RAID is currently a necessity to get persistent write cache.
I was recently astonished at the ease with which mdadm reshaped and grew an existing RAID 6 array from 8 to 10 disks. One command and it's merrily chugging away without the rest of the system even noticing. Thanks to the status-watching cron job it even emailed me when it was done two days later!
I think people take the md subsystem for granted, and that's a shame as it's one of the best things around on Linux systems.
(soapyfrogs.blogspot.com has contact info).