About a year-and-a-half ago, a study was released from Tel Aviv University that stated being multilingual helped fight off aging of the brain. While their report doesn't list specifics, it hints that even taking the time to learn new languages in adulthood (even if you don't reach fluency) can still provide those same benefits. Speaking more than one language basically provides exercise for your mental muscles. If you're interested in the full article, you can find it here.
More recently, a study published in Psychological Science in January states that being multilingual helps reading skills in your native language. The following sentence sums up the results of the study quite profoundly:
The findings suggest that after learning a second language, people never look at words the same way again.
Through eye-tracking studies, researchers found that bilingual (or multilingual) speakers took less time to process words that were cognates in their native and second languages. This study is remarkable because it turns the metaphorical table to provide insights to how speaking more languages can lead you to being more fluent in your own, native tongue. If you're interested in reading the full write-up published in Scientific American, you can find it here.
As I am writing this, my son is watching a bilingual show:
Both Dora the Explorer and Go, Diego, Go! provide dialogue primarily in English but have Spanish-speaking characters and include Spanish lyrics in memorable songs. While I am glad children in America can get at least a little exposure to another language at a young age (though watching the show will, by no means, turn those children into bilingual speakers), I find myself wishing for an adult equivalent--a TV show that features a character who speaks another language so that I can get input without being completely lost in the plot.
I hope that if you have never considered learning a new language, these studies might inspire you to try to pick up a new language, whether it's through sitting in on a class or picking up a language textbook or listening to radio shows in another language online or watching movies in a foreign language. If you are learning another language or have already learned another language, go ahead and thank yourself for taking care of your brain and allowing it to expand its neural connectivity.