Before launching into my list, let me preface it by saying that this is a list of what I consider relatively easy to use desktop distributions. So, while Debian is a great distribution, I haven't included it because to get the most out of it you should be an experienced Linux user.
Read them here
First, on my list is an old favorite of mine: SimplyMEPIS. What I like about this distribution is that, more so than any other Linux I know, it just works. It really does. I install it on any machine, it runs and life is good. I especially like that its creator, Warren Woodford, includes several small, but extremely useful utilities to make using the desktop easier.