The good news of using Ruby on Rails is that design your application is the most difficult part of the entire project. Some years ago I would have used Photoshop among  all the tools to draw effective web interfaces, but when I met Rails I began gradually to  change my mind. Rails drives you to the basics, teaching you that simplicity wins. Photoshop is awesome, but its simply  the wrong tool to design for the web; it's the right tool if you need to draw cool buttons or small details to include in your pages, it's probably the wrong one for the most part of the job. Then I tried to approach web apps design using bare HTML and CSS. It can work if you have already defined all the metaphors you would use in your application, but this design practice assumes that you have some good abilities in prototyping on paper and, most important of all, that you would be your own customer. 

In the case you need to show your work to other people, and they must agree on the direction you are taking before you can go farther, then you need a way to sketch for the web. HTML and CSS are good but you definitely need something different to try your ideas in seconds. This is especially true if you are in the business of consultancy: your customers need to be convinced by your visual analysis. 

Keynote, part of Apple iWork package, is definitely the best tool I've used right now to design for the web. While you design for the web you need something that avoids you dangerous art-like distractions. To design a web application you don't need to be an artist, you need to be an engineer.  Good taste helps of course, and probably is something you can improve during the years.

Keynote has everything you need: you can draw lines, draw squares and rectangles, use colors and shading. It's absolutely very good to work with fonts and text, and this is the quality I appreciate more than any other. It's really simple to get in, and it's cheap. 

Recently I found this wonderful tool has a way to lock shapes position on the slide pressing COMMAND-L, similarly to layers locking on Photoshop (this is something I couldn't found on Power Point). 

I really think this is all you need to accomplish quickly and effectively your job as a designer.