Free software detects what happens when you aren't there.
Syschat has an article on how to use a webcam as a security device to monitor unattended locations using Yawcam free software. The article outlines two ways to use software to record activity - one takes a picture at regular intervals of your choice and can store them offsite. The other one records activity when motion is detected.
Yawcam runs on Windows and needs Java Runtime Environment (JRE) 6 or later, DirectX 9 or later, Windows Media Player 9 or later, and Windows 2000/XP/Vista (some sites report no problems with Windows 7)
Either way, its a good use for a webcam.
Use Your Webcam as a Security Camera and Motion Detector
Another Yawcam tutorial can be found here.
There's also a web app that lets you monitor activity using an internet browser; there are three plans, one free and two paid. The free plan allows 1 camera, 50 MB storage, and 14 days of history. Ugolog Online Video Surveillance & Monitoring has no software to buy or install and has video history, motion detection, and alerts.