This was once a much longer list, but several dynamics have come into play. One is the movement of content to social media engines. Another is the attrrition of personal and university sites from the original days of the internet (but we're still here!) And a third is the congestion of some sites with so much advertising that it is unpleasant to visit them. In addition, some of the older sites are not HTTPS (secure) compliant.
Poetry Foundation - the online home of Poetry Magazine founded in 1912 by Harriet Monroe - this is one of the finest sites for anything on the Web. After receiving a massive ($200M) endowment, the magazine, which has permissions for a large and broad collection of modern poetry, created and developed a very effective site with rich content including biographies and extensive poem listings for hundreds of poets. (PC Rating: 10/10)
My Poetic Side - a very good site with a large collection of content and a large, interactive user community (PC Rating: 10/10)
The Internet Archive - super content rich, but you'll have to search for what you want. Many full volumes of poetry (and many other things); Librivox includes many recorded live poetry readings of excellent quality ((PC Rating: 9/10)
Project Guttenburg - the internet's orginal free library, and probably the gold standard for confirming what is in the public domain. P.G. provided the source texts for several of the books in this collection, but once again, you'll have to search to find what you want.(PC Rating: 9/10)
Poetry ut Loud - a popular site for online reading competitions with lots of content and activity (8/10)
Poeticus - a colorful, well-produced site with a fair amount of content and moderate advertising levels (PC Rating: 7/10)
Poetry Daily - a site with current content and discussion (PC Rating: 5/10)
Bartleby - originally an offshoot of Columbia University and now a commercial site, there is alot of content here, but the advertising content is getting pretty annoying. (PC Rating: 4/10)