You can get a whistle for about a buck in MANY places in my neck of the woods. Light, small, easy to use.... You just never know when you might need help.

A friend of mine slipped down a cliff at a local conservation area, while walking her dog in the winter a couple of years ago. She managed to grab a tree about 80 feet down her tumble towards the river another 100 or so feet below. The park is right in the heart of her city, and it was a routine trip for them, so she thought nothing of it as she left her car, cell phone and keys in her coat pocket. She was lucky enough to be able to use her cell phone with her broken arm, while holding onto the tree with the other.

She called her mom, who called the police, who called the local fire dept, who then called the regional rope rescue team. They knew what trail she took, but couldn't find her in the dark. Thankfully she had a whistle on her keychain, and they were able to locate her that way. $1.25 well spent!