If it is not possible for outfielders to find a clear, dry area to practice on (other field, parking lot, etc), try these indoor drills that work basic outfield skills. As always, if you have other drills, pass them along!
Pass drill: Each outfielder gets a ball and stands in line. One at a time, an outfielder jogs towards a coach and flips the ball to him. The player runs around the coach and goes out for a pass. The coach throws a pass with the ball and the outfielder runs it down. The next player than goes and the drill continues. This works on running technique, glove technique, catching on the run, keeping your head still when running, eye-hand coordination, and timing. After each player has gone twice, have players approach the coach from the other side so the player is running and looking over the other shoulder this time.
Corner drill: This drill replicates a ball hit to an outfielder's left or right. Players line up on one side of the gym (to the left or right corner of a base-line on a basketball court for example). A coach rolls a ball towards the other corner of the gym. The outfielder cuts the ball off (running down the base-line towards the opposite corner) before it hits the wall, turns, and throws to a middle infielder who acts as a cut-off man at the other end of the gym. This works proper angles, footwork before and after catching the ball, and hitting the cut-off man.
Zig-Zag drill: Outfielders line up next to and to the left of a coach at one end of the gym. A player runs straight ahead and looks over his right shoulder for the throw. When the coach yells "now" the player whips his head around to his left shoulder as the coach tosses the ball over that shoulder. The outfielder tracks it down and catches the ball. This drill mimics a ball hit over the outfielders head that curves or is wind-blown to the other side of the outfielder. Players should quickly whip their chin/eyes around to the other shoulder (yes, they will lose sight of the ball for a split second) so that they continue running with the same forward mechanics. After a few turns apiece, have players line up on the other side of the coach to practice the other side as well.
Come-up throwing drill: Players line up on one side of the gym facing the other side. A coach starts the process by throwing a ball at the far wall. The first player in line charges the ball that is now bouncing towards him, gloves it, and throws it to the far wall. The ball bounces off the wall and the next outfielder then charges and does the same. The round-robin drill can continue as long as the coach wishes. This drill works on charging balls, footwork before and after catching, being aggressive through the ball, proper arm angle (over-the-top) when throwing, and accuarcy.
First-step drill: Place four cones on the four corners of the basketball court. One outfielder stands in the middle of the court - the others off to the side. The player sets up as if a pitch is being thrown and a coach points to a cone. The player reacts as if a ball is hit towards the cone. A coach could even toss a ball towards a particular cone for the player to try to catch. This works on first step quickness to the ball, proper angles, and aggressiveness.
Note: All the drills mentioned above certainly work outside as well. Modify them for your needs and the ability level of the players you work with. Create some new ones of your own!