Watch a BitCoin account for incoming payments I want to watch a set of BitCoin addresses and be notified (preferably by email) when they receive a payment.

I prefer me not to write my own software for this and not to install it on my own server, but programming suggestions are welcome.
You need a server which indexes by address. Libbitcoin-server provides this index. All tx’s to or from an address of interest will trigger a notification to your subscribing client.

https://github.com/libbitcoin/libbitcoin-server/wiki

Of course you can always build your own address index.