I would like to develop solid web development skills. I have some back-end knowledge (my weapon of choice is RoR), but I am absolutely terrible with HTML, CSS, Javascript, all the front-end / user interface stuff.
Do you know any good resources to develop HTMl/CSS skills? Javascript is more of a programming language, so I don't think it really fits within this question, but if you know any good resources, I'll also welcome those!
Watch Douglas Crockford — The JavaScript Programming Language from YUI Theater to learn some javascript skills.
For HTML, i Learned a lot from HTMLDog. I don't think it is updated any more but it is all still relevant, as the HTML / CSS Spec hasn't changed since it was written.
Books I could recommend:
In my experience, most of the tutorials about HTML/CSS cover its sintax, tag meanings, attributes ,etc. You can cover them in a few days, and then you can forget quickly. IMO, it is better to look for designs, then see the source in order to know how they're made. In this way, you will be able to see what you can do with tags/css combinations.
You can see some free templates at the following sites.
First thing that comes to me, will be to understand the reasons of those meta languages, you should understand that the separation between contents, visuals, and functionnalities must be separated.
If you know RoR, you should know about the MVC pattern. It's quite the same here.
Try to learn, why, and you will understand how.
You should also know about (x)html semantic which a must to know before doing anything.
To have a good introduction, you could give a read to the Markup Guide which explains to clients every xhtml tags.
All links given by RegDwight should be a good start.
As a javascript library, i would advise jQuery, which complete, and easy to use, with a short learning curve (but complete anyway).
For CSS, your first stop should be Eric Meyer's site. He literally wrote the book (four or five, actually) on CSS.