pearDict allows you to quickly access various online dictionaries. It's integrated into your workflow. Select a word from any Cocoa-based app, then choose the 'Look Up on Web Dictionary’ entry from the Services menu. PearDict will search for the word. Or, you can move the cursor over a word to activate a keyboard shortcut and pearDict will automatically look up the word online. You can also use the same technique for current selections and text-cursor placements, with customizable keyboard shortcuts. This works even for non-service aware applications. You can also manually search for words using the pearDict search field. If you don't like the information provided by pearDict, you can always access the full-featured dictionary Web site with a mouse click.
pearDict is free and available in the hope that it will make communication between people from different cultures and languages easier.