Just to get one thing straight,someone said its about telling your clients what your business is all about,well its not.A logo on its own cant tell anything about a company,take Apple for example,if you see their logo,and never heard of the company in your life,would you think its a IT company ?Of course not.
A logo is important when creating a brand,something that will stay in peoples memory.Something that makes people remember what your company is all about by just seeing the logo
Using the same unique images that represent you and your products over and over on the web will instill these images into the minds of people that read your content and obtain your products.
So I do feel that it's important to establish an image or call it a logo or whatever that you associate with yourself and your business online.
I think a logo is the most important for any website, its something that the public can identify with and represent on official documents. It can be anything from a picture or the first letter of the website.
It will make or break your site, the best program to make logos in is adobe photoshop
And don't forget to copy write it
This is very important. A website has to be recognized for its uniqueness (if that's a word).
I've seen great sites that don't have a logo. And I've seen some of the absolute crappiest sites that have beautiful, intricate, well designed logos.
Logos are important ONLY if you're trying to build a brand and want your customers/visitor to associate some image with your business so that they will remember you. I don't see it at all as being necessary for a site to succeed OR look professional.