Read receipts are a nice feature of iMessage, allowing others to know exactly when their outgoing messages are viewed. On the other hand, for some privacy-minded iPhone owners, giving everyone this information might feel a little too invasive. iOS does give the option to disable the feature for all messages.
SelectiveReading takes things further, providing more detailed options under Settings -> Messages -> Send Read Receipts. The tweak offers to ask whether or not to send a read receipt every time, although this could result in many dialog boxes for people who receive a lot of iMessages. The real gem is the Contacts Can Override feature.