I've just launched BTC Watch. It's a simple service that monitors the Bitcoin network in realtime, allowing you to subscribe to real-time e-mail updates of transactions occuring on addresses you wish to monitor. You can opt to receive notification immediately when the transaction first appears on the network, and/or when the transaction has been confirmed between 1 and 120 times and at several steps inbetween.
You might find this useful for monitoring addresses you publish for receiving funds or donations, or for keeping track of your own addresses and transactions. No sign up is required. Just enter your email address, and the Bitcoin address you want to monitor, and you're set. Check it out at https://btc-watch.warrenguy.me. Some screenshots of sample email notifications are below. The emails have both text and HTML parts, and also include an attached file with a JSON object containing the complete transaction information.