There are many cases where the frequency is VERY important:
Anything with a timing circuit that relies upon line for timing
Anything with a linear iron core transformer (the volt-seconds applied at 50Hz is higher than at 60Hz even for the same voltage, power transformers are often designed on the very edge of suitable maximum flux before saturation so the additional volt-seconds CAN cause core saturation which might be a benign as the transformer getting hotter all the way to transformer failure)
Motors that are powered directly off of the line current.
There are 2 basic ways to convert 50Hz to 60Hz
1 is through a mechanical system that actually transforms the power mechanically through a rotary frequency converter:http://www.temcoindustrialpower.com/product_selection.html?p=about_frequency_converter
The other way (which is simpler by today's standards) is through a SMPS converter / inverter stage similar to what medik said: Convert to DC then invert back to AC at lower frequency.
It's not a trivial SMPS design but at 500W it's not that hard for an experienced power designer. I would not recommend it for a novice or even an EE who isn't quite good at SMPS and inverter design.
The best way to deal with this is to determine if you REALLY need a converter at all. If you have small motors they likely run off an internal DC bus and you very likely have off the shelf SMPS power supplies that could either be replaced, or that already can accept 50Hz or 60Hz. If you are talking about design I presume you can open the system up and verify the need.