A kickstand, shown in the left photo below, props up a bike without the need to lean the bike against something. The photo on the right shows a situation in which you would need a kickstand. The racks are too small to hold a bike upright.

Bar-ends, shown below, allow you to lean more forward when you want to assume a more efficient riding position or enjoy a change of posture. This is especially useful for bikes that put you in a more upright posture. You can have these added to almost any straight handlebar, such as those found on mountain, comfort, and hybrid bicycles.

For the downward curving handlebars seen on road bikes, aero bars offer a very similar function.
Both bar-ends and aero bars keep your hands away from the brake levers. This slows your braking response time in emergencies.
A cycle computer could be useful if you want to pace yourself or track your progress in a training regimen.

Common computer features are:
Some of the newer systems have additional features: