I have minimal need for a currency conversion site so when I do use them for either business or pleasure I want something that is simple and easy to use. I have used in the past a variety of sites and found that many are complex or you need to get through a number of screens before you get the answer you want. This is not the case with xe.com, which has now become the sole site I use for this.
First off, the name is great and very easy to remember. This might not be a huge deal for many but if you use a variety of computers (home, work, girlfriends house, parents, etc…) you need one you can easily remember. This does the trick nicely.
Once in, it is also incredibly simple to use and updates live. I think every minute or two it will update so you have the most accurate conversion possible. As you enter the site the home page is a simple to use table showing current currency values compared with each other for the major markets. This is a very handy at a glance tool and means you do not need to trawl through pages if you want simply the current value for £1 or Euro in $. True, obscure currencies can be more convoluted to find but if like me you tend to only be interested in the majors, this is perfect. At the bottom of the page you can do the calculation you require with a simple to use interface so little need to get deep into the site.
For simplicity of use, this is the outright winner for me and highly recommended.