There appears to be no cheat codes or hacks for Borderlands PC.

Fortunately, there is a save editor which you can edit your character level, money, items, skills are more!

Download Link: http://www.rapidcheats.com/game/borderlands/14/borderlands-cheats-amp-save-editor.html

Borderlands save file location:

C:\Users\Username\Documents\My Games\Borderlands\SaveData for Vista & Windows 7
and in My Documents\My Games\Borderlands\SaveData for Windows XP

If you want to edit your skill tree, you need to manually add the corresponding code for the skill, below is an example of maxed out siren skill trees

Happy Cheating in Borderlands!