I can recommend qstardict and its variants. I use it on my nokia n800 and computer desktops (kde/gnome) and it's very useful. There's also ports for other platf

I can recommend qstardict and its variants. I use it on my nokia n800 and computer desktops (kde/gnome) and it's very useful. There's also ports for other platforms. I can select a word (and optionally press a key combination) and it automatically searches the word in dictionaries. There's also plenty of dictd clients that do the same. The advantage is that it works everywhere, not only within the browser. There's also some firefox extensions and konqueror (KDE) can also easily do this by enabling the dictionary search engines, when pressing the right mouse button over a selected word.