From its origins in the 1970s—when it was used by climbers for practicing their moves in parking lots and eventually to bypass terrain that was hard to climb across—Slacklines have more recently also become a great training tool to strengthen your core and improve your balance or just fun to mess around on.
Gibbon Slacklines started selling a range of slackline kits in 2008. They now have 1" and 2" width slacklines geared for specific uses (the Jibline for tricks, the Flowline for long distances, the Funline for beginners). Slacklines are easy to set up between trees, poles or stands just about anywhere. All you have to do is wrap the area and winch the line on. Beginners can set them up close to the ground. Once you've mastered a few skills you can move the lines up to whatever height you're comfortable with. Slacklining has fortunately become popular enough to support competitions and World Cups.
Visit Gibbon to find retailers. Prices start around $75.