If possible, please include a link to the text if it's online, a mini-explanation of what the significance of the piece is that makes it worth the time investment (it's not very utilitarian to spend your life repeatedly reading the same arguments, after all).
Keep in mind that Utilitarianism Resources and Utilitarian Philosophers already offer pretty comprehensive lists of online writings and writings by the best-known utilitarians. While some links are bound to overlap, we can fill in the gaps rather than duplicating the pages. Neither page lists its links by topic (except by philosopher), so it might be helpful to do that here. To kickstart:
Very short introductions to different kinds of utilitarianism
Classical utilitarianism (aka hedonistic utilitarianism)
Preference utilitarianism - an alternative to classical util
Act utilitarianism
Rule utilitarianism - an alternative to act util
Indirect utilitarianism - a cross between utilitarianism and 'conscience-based' ethics
Prioritarianism - a consequentialist alternative that stresses egalitarianism more than util does
Glossary of more short descriptions
More detailed introductions
Questions for broadly utilitarian theories - an excellent short summary of the problems different forms of utilitarianism are supposed to address (
Primer on the Elements and Forms of Utilitarianism - a more detailed intro to a smaller selection of rival ideas
Utilitarianism FAQ and Common Criticisms of Utilitarianism - both addressing some of the more common objections
An Introduction to the Principles of Morals and Legislation, by Jeremy Bentham (available online) - in some sense the book that 'started' utilitarianism (can be quite heavy going, but you only really need to read the first four chapters to see most of Bentham's logic)
Utilitarian living
Practical Ethics by Peter Singer - a short and widely read book on topics such as abortion, euthanasia, animal welfare
Living High and Letting Die by Peter Unger - arguing that common moral reasoning (including but not exclusive to utilitarianism) requires the biggest sacrifices from us that we can persuade ourselves to make.
A Consequentialist Case for Rejecting the Right by Alastair Norcross and Frances Howard-Snyder - tempering (but not completely escaping) Unger's conclusion by arguing that utilitarianism is scalar; ie. there's no 'right and wrong', per se, only 'better and worse' (some of Norcross's papers are on his website, but not this one unfortunately). Publication details The Journal of Philosophical Research, Vol. 18; 1993, pp. 109-125.
What to maximise?
(PDF) Can there be a preference-based utilitarianism? by John Broome (who has numerous other papers online at his homepage)