I use this site for online chat. It has some nice features like spell check and reverse translate. You can change the direction of translation with the click of one button.
It will allow unlimited input but will only translate 1000 characters at a time. Fortunately the untranslated portion is still there, you just delete the first 1000 letters and continue.
Of course it works best with short sentences and can get confused with compound sentences (multiple nouns or subjects) but it does a pretty good job. It has really helped me with understanding Spanish, especially when you have to decipher what your amiga is trying to say.
http://imtranslator.com/default.asp