iOS (jailbroken): The ability to send read receipts in iMessages is great if you're talking with someone you actually like, but it's also a disaster if you're trying to avoid someone. SelectiveReading is a jailbreak tweak that allows you to control who sees those receipts.
By default, read receipts are an all-or-nothing affair. With SelectiveReading, you can pick and choose which contacts get them. So, if you have someone you talk with a lot and want them to have the piece of mind that you received a message, you can enable read receipts for them and turn it off for everyone else. If you're a heavy iMessages user with a love-hate relationship with read receipts, SelectiveReading is worth a look.