I have a pinterest board dedicated to useful teaching websites. Here is a list of my favorite ones to use with my English classes...
Poem Hunter will provide you with the poems from any number of poets. I use it all the time. Whenever I need to find a poem it is the first place I look. It has short bios on a ton of well known poets along with online texts of their writings. It is very handy. The site is super user friendly and comprehensive.
Prezi.com is a website that allows students to make cooler versions of power points. It takes them a class period or so to play around with it and figure out how to use all of the tools, but once they know how, they love it. They can add videos, pictures, objects, colors, themes and pretty much everything else that they can do with powerpoint. But, the visual effects are unique and fun to play around with. They can save them online and share them with anyone who has an email address. It is definitely worth checking out.
Purdue Owl is a great resource with a wide variety of writing tips. The material is most appropriate for students in middle school through college age students. I use it all the time as a reference for how to create bibliographies and citations using MLA formatting rules. Once my students have shown me they know how to format their bibliographic entries, I let them cheat by using easybib.com.
6 + 1 Writing Traits is a well known writing, teaching and assessment framework. The title refers to the traits in writing that we want all of our students to develop over time: ideas, organization, voice, word choice, sentence fluency, conventions and presentation. The common language is useful in aligning teachers across contents and grade levels to provide united expectations of students.
Finally, Grammar Bytes has a wealth of grammar activities to help reinforce grammar rules with yours students. The sentences are fun and contemporary.