Electricity Calculator
You should think more about how you arrange and style your code. Additionally, you should be aware of DRY methodology and keep from repeating code wherever possible. 

I made a couple of modifications to your code. Primarily, I cleaned up the arrangement and styling. After that, I added an Ohm class - which places all of your Ohm related functions as methods (again for code cleanliness):

"""
This class contains functions which perform Ohm's law and power 
law calculations based on variables from user input
"""
class Ohm():
    @staticmethod
    def voltage(i, r): return i * r

    @staticmethod
    def current(r, v): return r / v

    @staticmethod
    def resistance(i, v): return i / v

    @staticmethod
    def power(v, i): return v * i  

    @staticmethod
    def pvolt(w, i): return w / i

    @staticmethod
    def pcurrent(w, v): return w / v


After that, I changed how you output the block of text from a bunch of print statements, to 1 multiline print statement:

# First user interface menu. let's user choose between ohm's law and power law calculations.   
print """Ohm's Law Calculations:
    1. Voltage
    2. Current
    3. Resistance

Power Law Calculations:
    4. Wattage
    5. Voltage
    6. Current
"""


After that, I added a call to int() on the choice = raw_input(...) call so that all input is shunted to an integer. After that, we initialize our Ohm class to an Ohm variable. Then, instead of having multiple definitions for i, r, v,  and w - we ask if the choice is in a list, define i/r/v/w. This lessens the amount of duplicated code:

### Since we're asking for the current in multiple locations, simplify it
if choice in [1, 3, 4, 5]:
    i = float(raw_input("Enter current:"))

### Again, since we're asking for voltage in multiple locations, simplify it
if choice in [2, 3, 4, 6]:
    v = float(raw_input("Enter voltage:"))

### Simplifying input for resistance
if choice in [1, 2]:
    r = float(raw_input("Enter resistance:"))

## And lastly, simplifying for wattage
if choice in [5, 6]:
    w = float(raw_input("Enter wattage:"))


After that, we continue with an if/else block which calls the function according to the user input. Our i/v/r/w should already be defined in the block above. One major change here is that we are no longer printing directly from here. Rather we are setting the output in an output variable, which we later use to print.  

output = None
if choice == 1:
    output = "{0} volts".format(Ohm.voltage(i, r))
elif choice == 2:
    output = "{0} amps".format(Ohm.current(r, v))
elif choice == 3:  
    output = "{0} ohms".format(Ohm.resistance(i, v))
elif choice == 4:
    output = "{0} watts".format(Ohm.power(i, v))
elif choice == 5:
    output = "{0} volts".format(Ohm.pvolt(w, i))
elif choice == 6:
    output = "{0} amps".format(Ohm.pcurrent(w, v))  


Finally, we check if output variable is None. If it is not None, then we print it, otherwise we print an error. As for your 'funny' error, remember that error messages should be informative (ie, should tell the end user where the error occurred and/or why it occurred):

if output is not None:
    print output
else:
    print "Invalid input values, please try again"


Lastly, here is all of the code put together (and a REPL):

"""
Ohm's law calculator
"""

"""
This class contains functions which perform Ohm's law and power 
law calculations based on variables from user input
"""
class Ohm():
    @staticmethod
    def voltage(i, r): return i * r

    @staticmethod
    def current(r, v): return r / v

    @staticmethod
    def resistance(i, v): return i / v

    @staticmethod
    def power(v, i): return v * i  

    @staticmethod
    def pvolt(w, i): return w / i

    @staticmethod
    def pcurrent(w, v): return w / v

# First user interface menu. let's user choose between ohm's law and power law calculations.   
print """Ohm's Law Calculations:
    1. Voltage
    2. Current
    3. Resistance

Power Law Calculations:
    4. Wattage
    5. Voltage
    6. Current
"""

# Grab the user input
choice = int(raw_input("&gt;&gt;&gt; "))

# Initialize our Ohm class
Ohm = Ohm()

### Since we're asking for the current in multiple locations, simplify it
if choice in [1, 3, 4, 5]:
    i = float(raw_input("Enter current:"))

### Again, since we're asking for voltage in multiple locations, simplify it
if choice in [2, 3, 4, 6]:
    v = float(raw_input("Enter voltage:"))

### Simplifying input for resistance
if choice in [1, 2]:
    r = float(raw_input("Enter resistance:"))

## And lastly, simplifying for wattage
if choice in [5, 6]:
    w = float(raw_input("Enter wattage:"))

output = None
if choice == 1:
    output = "{0} volts".format(Ohm.voltage(i, r))
elif choice == 2:
    output = "{0} amps".format(Ohm.current(r, v))
elif choice == 3:  
    output = "{0} ohms".format(Ohm.resistance(i, v))
elif choice == 4:
    output = "{0} watts".format(Ohm.power(i, v))
elif choice == 5:
    output = "{0} volts".format(Ohm.pvolt(w, i))
elif choice == 6:
    output = "{0} amps".format(Ohm.pcurrent(w, v))  

if output is not None:
    print output
else:
    print "Invalid input values, please try again"