My Sheeva plus router is replacing two machines on my network for substantial power savings. Here is what I'm doing with the Sheeva so far as a home server:
1. DNS - router points all DHCP'd clients to the sheeva for nice internal network names, no editing of hosts files, and no need to speak in IP#s to share local content when friends come over.
2. Samba - The sheeva has a terabyte USB drive attacted to it with pictures, music, and backup data. Streaming data from it to other appliances is a snap.
3. openvpn server - router forwards a port to the plug allowing VPN into the internal network when away and access anything as if local.
4. openvpn client - always-on VPN to co-loc server where website hosting and other projects such as voice and game servers reside (that has substantial bandwidth, they say they have some OC192s coming into that datacenter
, so it's better than hosting at home). Router is configured to forward traffic to that IP to the plug so any machine in the local network can access the co-loc'ed server without extra software.
5. automated backups - various desktops and laptops are configured to sync to the plug daily for data backup
6. Wake-on-LAN support for my internal LAN. The Sheeva is always on but other computers may be off (in Suspend). The power hungry computers are configured to suspend (S3) after time and power buttons put machines in suspend. If VPNed in and need one of them on, the Sheeva can send the "magic packet" to wake them up, then remote desktop or any other remote management tasks can be performed.
7. Bittorrent client - Power hungry desktop need not be on anymore to download whatever. Just let the plug take care of it.
Idle power before Sheeva: 170 Watts (two machines, a BSD router and a Celeron file server, other desktops, laptops in suspend)
Idle power after Sheeva: 34 Watts (the plug, USB HDD, wireless router on, desktop and laptops in suspend, can still access all data on LAN through plug)
Projected yearly power savings (CA rate at my tier) about $150.00 !!
So the Sheeva pays for itself in under a year for me, a no brainer.