Many editors and IDEs allow the user to bookmark one or more lines and jump to them later. In Vim, this is implemented as its mark feature. For those of us who would like to visually see which lines have a mark, Vim offers nothing.
The vim-signature plugin is a perfect solution to this problem. Just install it using Pathogen and the marks you will place will be displayed in a new column to the left of your code. There are many other features of this plugin, but I think the visual marks is key.
Tried with: Vim 7.3 and Ubuntu 12.04.1 LTS