Aside from the obvious benefits of learning languages to understand other people and cultures, there is some evidence that speaking more than one language may improve overall intelligence. It should be noted that most of this research relates to people who have been bilingual from childhood, so it is not clear how this extends to adult learners.
See this article for information on one study.
To be reworked:
"When do People Learn Languages" is a fascinating article. I picked up on it because it was saying things I've been trying to tell people for ages: Kids don't magically learn languages; The FrozenBrainHypothesis? is almost certainly false. It's just that kids have all the time in the world to perform a terribly pressing need: To communicate with the people around them. But there's so much more in here! I highly recommend it.
One quote from the article: "Quite a few studies, in fact, find that adult learners progress faster than children".
I am surprised no one has created a web site with pictures, audio, or video of memory journey to use for the most common words in different languages. I have some old Easylearn tapes for Spanish from Learning Strategies (Paul Scheele) that use the concept of a memory journey in combination with mind maps in the booklet. The work is heavy on audi0 descriptions but very light on the visuals. It is easy to find a list of the most common words in any language. I would love to find a list of the most common words that includes a picture and their spanish (or german, french, etc...) equivalent.
Something like "The First 101 Words in Learning Any Language"?
I have just stated using the Link Word concept (by Dr. Michael Gruneberg) to pick up Spanish again. The LinkWord concept helps you learn commom words in a foreign language by linking the translation of the word to another english word, and prompting you to create a visual. For example, the spanish word for rice is ARROZ. The visual is several arrows flying into a plate of rice. I have gone through about 40 words so far, and had good success. But some of the connections are becoming more and more tenuous.
Barry F (5/25/07)
"Languages and cultures the fun way: Articles, essays, comments on learning languages" http://logoi.com/logoi_notes.html
_The Language Imperative: How Learning Languages Can Enrich Your Life_ book by Suzette Haden Elgin http://www.perseuspublishing.com/focus5.asp?ISBN1=0738204285
"Is English Learning Slowed Down with Bilingual Education?" seems to imply just the opposite. It implies that bilingual education gives students better English-language comprehension over the long term than English-only education.
"Anyone can speak 7 languages" "LEX Language Project is a multilingual education program where both children and adults can acquire several languages simultaneously." http://lexlrf.org/
Mudrast (30/04/2012) How I learned French in One Year 
Language-learning humor: ""LOL-Kitteh as a Second Language" :-).