There are a handful of free SMS services out there, but CallWave is my favorite. You can either install it as an OS X Dashboard widget, a Yahoo Widget Engine (Konfabulator) widget, or even as a Google Gadget on your Google Homepage (instructions here). If you use Windows Vista, you can install it as a standalone Vista gadget.
Yes, it works on Philippine networks. You can also send text to most other countries. And unlike most free PC to SMS providers I know, CallWave is pretty quick–messages are received in just a few seconds.
The Google Gadget version is pretty sparse and you have to regsiter your mobile number to use it. The OS X Widget has more features, as it can access numbers from your Mac phonebook for easy reference. I haven’t tried the other versions, though. But I’m pretty sure they’re all straightforward and easy to use.