Remotely turning on or rebooting a frozen computer
Server-grade computers by most major manufacturers have provided these types of management capabilities for many years. Watchdog timers in the BIOS, COM-based Telnet console redirect, IP based COM, remote KVM, etc.
For a desktop-grade computer you have a few options:
1) Replace your system with one that has these types of management features (out-of-band management).
Intel offers their AMT (Advanced Management Technologies) on the desktop as well, which when adhering to the certain requirement is branded as vPro. (Basically a vPro-compatible board and CPU).
AMD has DASH; with similar requirements I'm sure. I've never used it, and haven't seen any 3rd party support for it.
I prefer Intel's AMT personally. All versions (that are 'vPro') allow shutdown, power-on, reset (hard), as well as a textual BIOS and boot view. The newest versions include a VNC-based "KVM" that allows you interact with the boot process as well as the OS, so it can be used to diagnose boot problems (BIOS, POST, Bluescreens, etc.) as well as end-user and OS support.
2) Get a remote-controlled power strip:
As echoback mentioned, there's products by DLI (Digital Loggers Inc.):
I looked into this, and it was too expensive for small-time use. Perhaps the price has come down though. It was feature rich, but I've never actually used it.
Another option is Technology On Demand's recent iRemoteReset product:
It is aimed mainly for auto-rebooting a router/modem combo (we use them mainly for that), but it shouldn't have a problem dealing with a desktop computer as a load. You can hit it by HTTP and reset the sockets manually (individually), as well as have it auto-reset due to lack of Internet connectivity, schedules and such.
3) Build an Interent-enabled robotic arm.
:)
There's an option in sysdm.cpl -> Advanced tab -> Startup and Recovery section (settings) -> called "Automatically restart" that will automatically restart the machine upon a blue screen.
Machines that depend on remote access for configuration/support need to have this enabled. You can then check eventvwr.msc or use a utility like NirSoft's BlueScreenView to later determine the STOP code and find why it crashed.
There are many types of freezes. A "hard freeze" where even the mouse stops working has a high percentage of defective hardware as the cause and needs to be physically looked at when that occurs. I've had users say their systems are frozen, but the system is just "stuck" at some point and they can still move the mouse. I've been able to use Sysinternal's PsExec sometimes to get a remote command prompt and issue a shutdown -r -t -0 command which reboots it.
At my place of work, we are a Dell shop. A lot of newer "high-end" Dell computers (T5400 and T5500 specifically) have a feature called AMT (that we don't use for some reason...). Supposedly it is a component that lets you remote reboot (even if the system is hung or powered off) and access the BIOS remotely. I don't know if you can get something with similar capability on a PCI card. I'm sure other manufacturers have a similar feature in their higher-end workstations and something like this is likely what you want.
I haven't found a way to reboot a computer that is hung up besides a remote power switch device. I use this one ip-p3 and it supports rebooting over the LAN or telephone. It's $200 though.
If you need something a little simpler, you could check this one out this. It's only $100, but the features of the IP-P3 are a lot better.