Tool that generates electrostatic maps of molecules?
Most computational chemistry software packages geared toward students/education should be able to do this. Examples include Scigress, formerly CAChe, Spartan, which has a 'lite/student' version that can probably do it, etc. Generally, any modeling software, for example, Gaussian or GAMESS should generate a file with the electrostatic potential map provided you give it the correct instructions. The output files of these methods can be visualized using many types of software (like Scigress and Spartan) and open source software like Avogadro. 

If you are looking for an inexpensive solution, GAMESS is free for research/education and Avogadro is open source.