A couple options for you:
(1) Camcloud - web-based service that connects webcams via your browser. Has motion detection, live streaming, few other features. Should be good for what you described.
(2) Vitamin D - software package that you install and configure. Lots of features and very powerful, but might be overkill depending on how much tweaking you want to do.
Worth taking a look at both see which one works best for you.
Anyone know of a way I can take my existing webcam and use it to monitor my home while I'm away?