There are lots of online sites that offer free brain training games. The Alzheimer's Disease Institute is a good site, as is Brain Metrix. These sites contain a variety of games which will test your memory, creativity, word association and IQ. Many sites also offer free programs that say they will improve your memory and thinking power over time.
Sudoku is a popular number-placement puzzle that requires the player to fill out small grids so that each column, row, and box contains the numbers 1 to 9, usually within a certain time limit. The puzzle is used in many brain-training programs to improve a person's response time and processes of logic and deduction. It can be found for free online in sites including Facebook and also in most newspapers and many magazines.
Dr. Kawashima's Brain Training is a popular software used with the Nintendo DS. It was developed with the help of Professor Ryuta Kawashima to train your brain in just a few minutes each day. The program includes an analysis "brain age," puzzles, riddles and tests that become progressively more difficult over time. The program claims to improve your thinking speed and memory if played daily.
A great way to kick your brain into action is simply to do your everyday activities with your non-dominant hand. For example, if you brush your teeth with you right hand, use your left hand. Cook with your opposite hand, answer the telephone, drink a coffee. Your brain establishes neural pathways for your "regular" behavior. Thus, making small changes forces your brain to make new connections and use that are usually less active.