I got thinking about this last night after enabling it as a Zarafa mail-server - exactly what it does. It's only after stumbling upon this topic that it made me write it all down!
I can't believe how much I have managed to squeeze out of a simple NAS device.
My QNAP is legitimately used as/for:
-Central storage (obviously!)
-DHCP server (via dnsmasq as I want the leases given linked to the DNS server below)
-DNS server (via dnsmasq too - so that i can browse the web-sites and pick up mail/browse calendar on wireless inside the flat and out without having to change where to point to)
-VPN server (thanks, qnap, for the latest firmware update!)
-FTP server (occasionally give access to my sister for files)
-Syslog server (consolidates logs from the QNAP, my firewall/router and my IP Camera)
-Xdove SMTP server (used to notify me of any syslog warnings or errors, outbound smtp for the Zarafa Mail server and used by the Camera to alert me to movement - attaching JPG stills)
-Zarafa Mail server (used for shared calendar access rather than emails. Both myself and the Mrs use z-push on our smart phones pointing at the QNAP to share a calendar.)
-Web server (3 web-sites - 'Gallery' for my photography hobby, 'Wordpress' for the Mrs's blogging, and an old HTML site i created years ago for posterity)
-MySql server (managed by MyPHPadmin and houses databases for Gallery, piwik, Zarafa and wordpress)
-Backup server (multiple backups to external hard drives for about a month's worth of retention)
-Remote replication server (used to mirror my data to my dads qnap on the other side of London, and his data to mine acting as an off-site backup for both of us)
-iTunes server (for the Mrs's iPhone/iTunes!)
-UPnP media server (for access by my Windows Mediacenter and Xbox)
-Photo gallery (specifically used for friends and family photos for publishing)
-Bit-torrent client (with the QGet installed on my PC as a front-end)
-Surveillance station (on constant record in case of a break-in!)
-WOL tool (I put my computer in standby to save energy. Can't send WOL over the internet, so I ssh into the QNAP and use the installed IPKG wol tool to wake up the computer before using RDP to connect to it. All of that can be done from my WP7 too).
I think I've got my moneys worth out of my TS-410.
Still looking for new ways of using this QNAP though!