For simple, on-the-fly kitchen conversions, type something like "4 tablespoons into ounces" into Google, Bing or most any search engine. For weird units and precise measures, consider ConvertCenter, a dedicated webapp.
Click on the general category you're looking to convert in and around in the left-hand column, and then choose the first measurement you're offering up. ConvertCenter instantly refreshes with the units it can convert to, and you get your answer. That's in "Classic" mode, at least—"Advanced" offers quite a lot more data on the conversion you're making. If ConvertCenter seems like it's just too tricky an app for what you're trying to do, the developer threw a little Google box at the top for you to try it out there.
Free to use, no sign up required, and seems like it's worth the bookmark.