I use AltaVista Babel Fish and Google Translate fairly often and am annoyed that both require me to specify both source (text to be translated) and destination languages. The former could be guessed at from the input text and the latter trivially obtained from browser settings (Google at least defaults to English destination at google.com and Spanish at google.es).

LazyWeb, failing AltaVista and Google fixing this, someone should write a Greasemonkey script that does.

Comments at this article point to various language detection techniques.