I was originally using Trulia to send me emails on new homes in the market, and it works fairly well. Problem is that the data can be sort of dated fairly quickly as it doesn't get MLS data immediately (at least in my area). This is sort of troublesome in markets where a home can be sold in 48 hours from being listed.

Someone on HN mentioned Redfin, and I tried it out. It's incredibly fast (guessing 24-48 hours quicker than Trulia) to get emails on new listings as it's using the MLS data directly. Give it another year, and I feel like the UI will be on par with Trulia on the desktop.