I'd like to know if I can put lists on the same vertical line in Trello. I couldn't do it but it would be pretty good for my work. Some lists don't need to be separated in different columns.
Update - 2014-07-29
The Trello design is no longer responsive, i.e. it no longer resizes as windows shrink. Trello support, in Case FC2725939, told me it is by design:
You don't mention what platform or browser, but the article Building trello.com for multiple devices describes how Trello is laid out, and so how you can control the layout.
So, if you create a narrow browser window, and/or increase the font size, it should eventually switch to a single column view.
Here is a series of screenshots, showing Trello in gradually narrower and narrower windows, showing how it rearranges its contents to fit the space available.
You can also use this Chrome extension I've recently published: Vertical Trello - Chrome Web Store