Most sites with poetry tend to be filled with ads and poorly designed. Hello Poetry (right now at least) has no ads and has a nice, simple design. You can browse classic poems http://hellopoetry.com/browse/poems/classics/
Keats http://hellopoetry.com/-john-keats/
Blake http://hellopoetry.com/-william-blake/
As well as contemporary poets http://hellopoetry.com/browse/poets/contemporary/

The only knock against the site is that it's incomplete (no Shakespeare?) but the site looks new, so I hope they continue to add to their collection and keep up the clean, ad-less design.