In the US, electrical outlet frequency is listed at 60Hz and 120v. If I am traveling to a country with 50Hz, do I need a transformer? My appliances work with the voltage that is listed in the country I am going to, but I don't know how frequency plays a role.
It's not as important as the power requirement.
From a piece on Wikitravel:
Frequency is generally not a problem--most travel items will work on either 50 or 60 Hz. If all the electrical appliance does is produce heat or light (except fluorescent lighting), then the frequency is unlikely to matter.
However, I assume you're talking about a laptop or phone or something. Some devices come with a frequency switch, others that depend on timing from the frequency (clocks) may experience issues with timing. But in general, as per the quoted paragraph, you'll be fine.
I decided not to quote the entire section, there's quite a bit more to read at the link on Wiki if you're interested - Voltage and Frequency for Electrical Systems.
Similarly, from the same page is a section on Applicances - it'll cover the common ones from laptops, radios to cellphones and cameras.
Check the description on your device to see what it accepts--most things say (check the wall wart rather than the device if it uses one.) To date the only thing I have found that had trouble on 50hz power was a clock--it was running at 5/6 the speed it should have. Note that some motors will have the same issue.
Almost all travel electronics already support 110-240/50-60 power and aren't an issue anyway. The only item I carry that isn't universal is my toothbruth and it doesn't mind 50hz rather than the 60hz it was built for.
The simple reason is that a converter does not affect the frequency. You would need a rectifier-inverted-rectifier combination to transform 50Hz to 60Hz or vice versa.
While technically possible, I've never seen such a thing in my life.
Also, the frequency usually makes no difference either way (most devices run on DC internally anyway).
Most devices don't care about the difference between 50 and 60 hz. The main exceptions being some clocks which use the mains frequeny to track time and "white goods" appliances which use synchronous or induction motors whose speed is closely tied to frequency. Vintage audio equipment can also be a problem due to the use of motors whose speed is determined by line frequency.
It is usually cheaper to replace the device or at least the motor than to convert frequency. A transformer will not do it.