"Messier45.com constitutes a non-commercial effort to provide the deep sky observer's most valuable tools online; the map and the lists of objects. Great effort is made to make accurate data accessible to both the novice and experienced observer."- According to the website itself.
I stumbled upon this website on Halloween morning. From the start it peaked my interest since I'm really a deep sky fan.
I really like the Deep Sky Browser. A user has to register (free of charge) but this is a powerful too for planning. It is basically an online database that presents the information if a spreadsheet type of layout. It gives the RA and Dec, Magniture, Constellation, Type, Classification, Size, and SkyAtlas Number for each object as well as many much more user selectable information. If you wanted to get more details on a specific object you can click it and it will show a detailed page listing all the other names of the object, and even 3 star charts to help you star hop there. Also provided is a photograph of most objects so you can verify what you are looking at.
The Messier Gallery is fun to look at. It shows pictures of each Messier Object. There are dozens of sites out there that have this but this is by far the best I've seen.
There is a Hershal 400 section for all those Dim Objects. The Articles and Guides section has some good reading.
Go out and take a look at this website I think you might want to bookmark it.