This is intended to be helpful for readers who are new to the trickier end of the poetry spectrum. I've listed the web resources that I've found useful in my attempts to become more familiar with difficult work. These resources are therefore entirely subjective and are not in any way definitive. I've avoided material that isn't available on the web and hope eventually to produce resources that may be useful for specific poets.
OED The Oxford English Dictionary is essential because many of these poets rely on secondary meanings and of the etymology of particular words. Access via your local library is free- if your service doesn't provide this then you can join a neighbouring service that does.
DNB. The Dictionary of National Biography is less essential but does give a reasonably balanced view of the poet's life and the the context in which she worked. Access is via the same route as the OED.
MED The Middle English Dictionary provides a range of definitions for each word. I couldn't be without the web version when paying attention to Langland, Hoccleve et al.
Google. As webmasters are getting more proficient at attracting specific audiences, so has Google become better at responding to those audiences. It is, however, a little ropey and continues to list those sites that are good at optimisation higher up rather than those that are helpful. A recent search for 'alliteration' produced all top ten results ( except the ubiquitous Wikipedia) that were providing definitions rather than more detailed explanations.
Wikipedia. Is much more detailed and reliable than it was but depending on volunteer contributors does result in some skewing. The 'alliteration' page concentrates on the Americans and can only be described as superficial whereas the 'alliterative revival' page provides an excellent overview and further pointers to what I was looking for.
Archive of the Now. The archive provides sound recordings of contemporary British "innovative" poets reading their work. A much improved resource but with surprising ommissions and some residual clunkiness.
PennSound. An aural delight featuring recordings of the best of American poetry over the last hundred years.
UbuWeb. Professes to be "All avant-garde. All the time." but seems to veer towards conceptual and 'visual' work. I find it tricky to navigate..
The Claudius App Easily the best poetry zine on the web with contributions from most of the best on either side of the Atlantic. The navigation, however, gets more infuriating with each issue. In the lates issue readers need to place a letter in the search box and names of poems and poets will appear
TEAMS Middle English Texts Series". This important collection of Middle English verse also provides a helpful and detailed introduction to each poem.
Jacket One was the most useful site to me when I started to pay attention to this material both in its scope and tone. The One archive is currently held on the Jacket Two site which seems to consist of quite intense conversations about Not Very Much.
Geoffrey Hill. Hill's collected essays are, as you might expect, densely worded but full of insight and glorious idionsyncrasy, subjects range from the obscure (T H Green and Henry Wooton) to the more familiar (Ben Jonson, Gerald Manley Hopkins, Ezra Pound). The Language, suffering and silence and Poetry as 'Menace' and 'Atonement' essays provide valuable insights into the way Hill thinks about poetry in general and his own work in particular.
J H Prynne. All of Prynne's, as yet uncollected, essays provide some insight into his work. I've found his Difficulties in the Translation of "Difficult Poems from 2010 to be the most helpful in terms of providing suggestions as to how his work might be read. He's also published extremely detailed readings of Shaespeare's Sonnet, 94, Wordsworth's Solitary Reaper and George Herbert's Love III which are similarly helpful and also provide examples of the kind of readerly attention that great poems require/demand. Most of Prynne's essays are available from various sources online, the Shakespeare and Wordsworth books can only be obtained from second hand sites.
Ezra Pound. I know that Pound produced much criticism but I only want to mention here his 1913 essay A Few Don'ts which is a list of things that poets should/must avoid. It's written by someone who clearly knows a great deal about poetry and I agree with (almost) every single word. It can be found in the Contributions to Periodicals 1902-14 collection which I obtained from the Internet Archive.
Simon Jarvis Simon is Gorley Putt Professor of Poetry and Poetics at Cambridge University and has written extensively on the role of prosody in the poem. His Wordsworth's Philosophic Song (2007) makes the case for metrical and rhyming verse as a way of 'doing' philosophy- his longer poems are, in part, a demonstration of this. There are many of Simon's essays available on the interweb but I've found his Why Rhyme Pleases as the most helpful when paying attention to his poetry. There is also a video, Bedlam or Parnassus: The Verse Idea of a paper Simon gave which uses Pope's Essay on Man as an example of how prosodic constraints can make the philosophical poem more effective.