I have paid versions of both Baylon (on a PC) and Ultralingua on a PCs and my Macs. I prefer the Ultralingua suite of language tools that have been around since time immemorial. Babylon is so similar that it is rumoured they copied the original idea from Ultralingua. I find Ultralingua, faster, better contextual content,and of course much cheaper. For those that don't want to pay and download it, the lexical content is available free on-line. I've been using it on Mac since 1999. but my preference for it might just be a question of choice and what I am most used to. I've been a professional translator for nearly 40 years and have tried and tested almost everyhting there is. One "program" to avoid is the Whitesmoke, which as its name suggests, is nothing but smoke - a vertiable scam. Like buying something in a box and when you get home the box is empty and the shop refuses to refund your money.
Ultralingua don't pretend that the software is free but it comes with a fully functioning ten-day free trial. They also make API and Ajax solutions for forums and software. I teach linguistics, and part of that involves teaching people how to use dictionaries effectively.