During gameplay, press [Shift] + [Enter] to open the console. Then type any of the following codes:
blitzkrieg - Skip Level
instant delivery - Get Units Fast
smarten - Increased Rank of Selected Unit
mo money - Get Additional 1000 Credits
quicker than death - Kill Enemy Units with 1 Shot
show fps - Show Frame Rate and Polygons (in v1.3)
On the first level, enable the mo money code. Unlike other levels (excluding the last ones), you will have every vehicle unlocked. After the second level, you have to unlock them yourself.