Insurance when coming back to the states?

How does the insurance generally work when visiting other countries (coming home to the states, other EU countries, etc)? Does NHS cover anything that happens abroad? Do you guys typically get travelers insurance just in case (this would be for multi-week visits)?

Thanks!

For other European countries you can get an EHIC card for medical.  It's free.
Obviously that won't help if your bags go missing or something else happens though, so travel insurance is a good idea.
For anywhere else, you need travel insurance for medical emergencies.

http://www.nhs.uk/nhsengland/Healthcareabroad/pages/Healthcareabroad.aspx

The NHS isn't an insurance scheme so it's coverage doesn't extend outside the country.  You can arrange to have the European card (as mentioned above) which works on an exchange system, (ie you get the basic medical care for the country you're visiting) but for some European countries which have very basic nationalised care it's still well worth taking out holiday insurance cover when you visit (it's not expensive and you can get just a week's worth if you want).

For the US you'd need to arrange medical insurance cover.

You can get worldwide travel insurance policies that cover the US.