Anthony Skelton has a good article at The Globe and Mail, while Roger Scruton reflects critically on Mill's views at the Wall Street Journal.
The SEP article on Mill, by Fred Wilson, is worth reading as well. Much of Mill's work should be contrasted with that of William Whewell, Mill's major philosophical opponent.
Mill was a prolific writer, but an important sample of Mill's works can be found online:
Auguste Comte and Positivism
Considerations on Representative Government
The Contest in America
Essays on Some Unsettled Questions in Political Economy
The Principles of Political Economy (Book 1)
Three Essays on Religion (first two essays)
The Subjection of Women
However, the Online Library of Liberty has done the rather hefty work of putting all 33 volumes of Mill's Collected Works online in PDF.
One of my favorite passages in Mill has to do with my own field of early modern philosophy:
Every tiro in metaphysics is familiar with the name of Berkeley, and thinks himself perfectly well acquainted with the Berkeleian doctrines: but they are known, in most cases, so far as known at all, not from what their author, but from what other people, have said of them, and are consequently, by the majority of those who think they know them, crudely conceived, and their most characteristic features misunderstood. Though he was excelled by none who ever wrote on philosophy in the clear expression of his meaning, and discrimination of it from what he did not mean, scarcely any thinker has been more perseveringly misapprehended, or has been the victim of such persistent ignoratio elenchi; his numerous adversaries having generally occupied themselves in proving what he never denied, and denying what he never asserted.
From here (PDF). Too true, even today. Berkeley is just not properly taught. (But Mill doesn't move us forward much, I'm afraid. For instance, Mill later in the same essay makes the odd claim that Berkeley probably knew no freethinkers or freethinking arguments in writing Alciphron. But Berkeley is very clear that the freethinkers he has in mind are coffeehouse strongheads, and he would have had plenty of opportunity to come across such creatures whiling he was hanging out with Jonathan Swift and Richard Steele. It's a common error: Mill is right that only a handful of people had tried to set out the case for the atheistic side if you are thinking of canonical or near-canonical works. But it does not follow from this that it never came up in coffeehouse arguments; Berkeley says it did, and what he says seems to fit with other things we know about coffeehouse philosophy, as well as fitting with the complicated mix of arguments he sees the need to answer. This, alas, is an error that plagues history of philosophy: we rely so much on standard and semi-standard texts of influential thinkers that we forget that these are only one carefully crafted product of philosophical thinking, and that there was philosophy being done in the period that did not take this form. See also the SEP article on Anthony Collins; Collins was one of the coffeehouse 'minute philosophers' that we know Berkeley had in mind.)
Other posts for the occasion in the blogosphere. Posts added as I find them. If you come across interesting ones not listed, let me know in the comments.
Mill at 200 at "The Elfin Ethicist"
Happy Birthday JS Mill at "Matthew Mullins"
John Stuart Mill at "I Want to be a Muso"
"Catallarchy" is having a Mill-fest for the occasion. You can access it through the introductory post.
Thanks to Matt of "I Want to be a Muso" for pointing out that the BBC (Radio 4) has a special program devoted to Mill, which can be downloaded until May 25 and afterward should be streamed online.