For many programming languages we still like to use either 80 or 120 columns in our editors to ensure it fits easily on print, as well as to use it as an aid for ensuring concise code.
In Sublime Text you can set vertical rulers for this by going to
Preferences » User File Preferences and add
rulers 80 120 and save the file.
For Sublime Text 2 it’s under
Preferences » Settings — User, but the configuration file is now in JSON format, so you need to add
"rulers": [80, 120] and maybe you need to append a comma at the end if you have more configuration directives following it.
Addition 2013: in Sublime Text 3 it is still under
Preferences » Settings — User and the file is still in JSON, so simply add
"rulers": [80, 120], like in the example for Sublime Text 2.