Staying organized on set is essential, and using a slate is a great tool for just that. The less time it takes to edit your video, either by syncing footage or finding the take you need, the more money you save. If you're a one person show, the more sleep you can get!
Using a production slate is an inexpensive way to make your life easier when doing video work. It's a small step on set that lets you easily sync audio and multiple cameras. It also reduces the headache of whoever has to edit your video when you note "1.4 A, take 3 was the best shot" and that person can quickly scrub through footage and find the slate that reads "1.4A, take 3".
A real slate with a time code can cost well over $1000, but you can pick up a basic slate almost anywhere for about $30. If you have an iPad, you can use the free app called "Clapperboard". (No link available to the app through the browser, but a quick search in iTunes on your iPad should show it as the first result.)
There are a lot of apps out there with more functionality, but if you're looking for the best of the best, you can't go wrong with MovieSlate ($30). This is a very robust app that in addition to a clapperboard, gives you a shot log, that allows you track camera settings, metadata, and anything else about a shot you might need for editing, reshooting later during pick-ups, or special effects work.