From the Invisible City Productions website:
Tumblewords: It's like a cross between Scrabble and Connect Four.
On your turn, Place a tile from your hand in a vacant space at the top of a column. That tile 'falls' down the column until it reaches the bottom or lands on another tile.
Look for words that can be formed that use the tile you just played. Words can read up, down, left. or right, but must be in a straight line. Score one point for every letter in each word you find; palindromes count twice. If your tile set has blank or 'wild' tiles, declare what letter the blank represents when you play it. That tile continues to represent that letter for the rest of the game.
When you feel that you've found all the words you can, draw a tile and end your turn. Play passes to your left.
The game ends when there are no vacant spaces on the board.
Board and rules are available here:
http://www.invisible-city.com/play/119/tumblewords
Online Play
Super Duper Games