Tools that cost moneyBlackboard: This is a great tool with a wide-range of functionality and a help-desk to contact when you have problems. The problem is it isn't free and would require a district-wide subscription making it unrealistic for a single teacher to purchase and use.
Elluminate Live!: This is similar to Blackboard in the range of features and functionality. The one difference is that you do need to download the Elluminate Live! software and host it on a server.
Tools that are free but require a downloadMoodle: This is an open source option that many schools and districts are adopting. The benefits of open source is that it is free and can be modified. The downside is that it doesn't come with a built in help-desk or support department. However, there is a great network of support around the Moodle environment that you can access if you are interested in using this tool.
Claroline.net: This is another open source tool similar to Moodle that requires software to be downloaded and hosted on a server. The best way to learn more about this tool would be to play around with the demo classroom they have or look at their tour.
Tools that are web-basedChalksite: There is a free version of this tool that is fairly comprehensive. The downside is that only 5 students can subscribe with the free version. For a monthly fee you can upgrade to different options that give you more features and allow more students to use the system. They have a series of screenshots you can view as a tutorial of sorts through the environment.
HotChalk: This is a free tool you can use to create your own virtual classroom. They are affiliated with NBC so you will notice that logo on the site here and there. They have a nice video tour you can watch to learn more about this environment. They provide access to digital content from the History channel, National Geographic, NBC News Archives on Demand, all of which seem to be great additions but they cost money so be aware of that when considering how you might use this content in your site or courses. You can access lesson plans and other content created by the community of users (other teachers such as yourself) for free and use that content in your teaching.
Ecto: This is a free tool that is a cross between a social networking site and a virtual classroom. There is a large emphasis on allowing students to collaborate and on being able to access content generated by other users and instructors. As you can see from the video below, Ecto is being used for both online courses and for supplementing face-to-face classes. The video provides an overview of the features and also some user testimonials for this product.
edu20: This is another free, web-based option that has a huge range of features and tools. You really can't explore any of the features without first creating an account but this would definitely be worth the time to check out considering all you can do with it. The screenshot below shows the list of available features. There is a nice tour you can go through but you need to be logged in to see it, which means you need to sign-up for an account first. Did I mention the account is free?
NICENET: NICENET is run by a nonprofit and when you look at it, especially after seeing HotChalk and edu20, you may think it seems a bit boring or bland. I'm guessing NICENET doesn't have the same kind of financial support that the other tools have and so it has placed it's emphasis on providing a decent set of features in a logical and easy to use environment and not on flashy icons and colorful layouts.
WiZiQ: WiZiQ is a bit different from the other tools that have been reviewed because it is more about actively teaching and presenting information online to students. This tool would be good to use if you wanted to teach or demonstrate something to students in real time but it doesn't have the same range of features as the other tools to support a long term online course where you have assignments, regular discussions and so forth. WiZiQ has an interactive whiteboard you can use with students as well as real-time audio and video chat capabilities. This might be a good tool to use in conjunction with one of the other virtual classroom tools when you want to hold an online meeting with students to go over something. You can check out their tour to learn more about this tool.