GREAT FOR COOKING:
Cooking Conversion Online -I use this one all the time. It is an excellent food specific metric converter (as long as you can look past the advertisements!)
WeekendBakery – A nice cooking conversion tools to quickly convert your favorite baking ingredients.
BBC GoodFood – Besides the nice designed website you have access to tools like usual measurement converter, a guide on oven temperatures, cake tin sizes, and sugar temperature guide.
Food.com – They have a neat food conversion calculators that can be usefull.
Epicurious – A part of beign one of the top recipe websites they also have a nice food measurement chart that shows you the multiplication factors for all metric equivalents.
Cooks.com – From this page on Cook.com present you can find a lot of information about cup measure or how to measure accurately.
Diana’s Desserts – Diana Baker Woodall’s personal site is focused on baking, but it has a conversion calculator which includes butter measurements too and one for temperature between Celsius and Fahrenheit.
Kitchen Calculator PRO – Cooking Conversion Calculator Apps for iOS
Worldwide Metric – General conversion chart (not food specific)