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:
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.
Also, the frequency usually makes no difference either way (most devices run on DC internally anyway).