[Wolf] sent in this site a while back, and I thought that this frequency meter how-to looked pretty useful. It’s based on an Atmel AT89C52 and a gutted multi-meter. The circuit isn’t too bad.

If, like [Giovanni], you want to keep better track of your usage, you might find value in his DIY energy meter build ... current, frequency, and power factor, so you can really nerd out over ...