Have you checked out Windows Live Translator Beta yet?
I absolutely love the UI features! My wife and I have been all over Latin America and I everywhere we go I pick up a local newspaper everyday and in the evening I kick back with a cerveza or two and my "dictionario" and see what sense I could make out of what was happening in Mexico/Guatemala/Cuba/Dominican/Costa Rica... So for me, translate.live.com is awesome as it's not just copying and pasting text like BabelFish or Google. You can manipulate the UI in a bunch of interesting, and very helpful, ways. Here's some screenshots of what I mean:
Head over to http://translator.live.com
Then, as you'd expect, you can simply paste in the text if you'd like, select the to/from language and hit "translate". But here's something better....
Notice the "Translate a web page" portion at the bottom? In this case, I pasted in one of the newspapers from the Domincan Republic. Hit the translate button (and ensure you've selected "Spanish - English" in this case) and you'll see that one of the views you can choose is a side by side layout that allows you to hover over text to compare the original with the translated. Awesome!
let's try one of the other views... I really like the "Original with hover translation" as it leaves the page in Spanish, and only when I hover over the text does it translate.
Notice the highlighted Spanish text and the English translation above it?
You can also do the reverse and see the entire page translated and then hover over with your mouse to see the original language.
What do you think?