I own a lot of cookbooks that are published in the world of pounds, ounces and farenheit (mostly the U.S....British cookbooks nowadays have metric or both metric and imperial) and the rest of the world, which uses metric. I also read various web sites and food blogs from all over the world. Converting units from one and the other can be a bit of a bother, so I try to include both in my recipes. I am guilty of using American cup measurements sometimes, but I try to limit that to recipes were the amount doesn't have to be totally exact, such as for bread. 1 American cup is about 240ml if you need to convert.
Most cookbooks and sites don't have both kinds of measurements though. I used to have a link to Online Conversion  on my sidebar for doing conversions. This is a great site where you can convert just about any unit to anything else. But the past couple of years I've not had the need to go there, since most common conversions can be done with Google Calculator, typing easy queries into the Google search box.
For instance, if you enter "250 degrees c in f" in the Google search box, you'll get an instant answer: "250 degrees Celsius = 482 degrees Fahrenheit". "150 g in oz" yields "150 grams = 5.29109429 ounces". You can even enter something like "3.5 us cups in ml", and get back "3.5 US cups = 828.058831 ml", and so on. And if the calculator can't figure it out, you get the usual page of search results. (For international shoppers, it also does currency conversions. Very handy when I'm going on an Amazon jp/fr etc. spree)
It's really convenient if you use Firefox or Safari as your browser (or the new IE7), and just enter it in the Google searchbox in the browser bar. Give it a try next time you encounter a site with measurement units from the other side!
(I was recently showing something online to someone and was surprised that they didn't know about this great Google feature. So...here it is, in case it's new to you too.)