Yahoo! Towers(tm) is like Columns, with the following additions: Up to eight people can play, in teams of two, and there are "power" blocks that can be stored up.

The way team play works is that there is a right player and a left player. Their play area are next to each other, and you can make matches across the play areas, so say we had something like this:

RG  | BBY
OWP | ORG
BPB | RGR
---------

and a peice with a B fell on the left player's side, she could drop so the board would look like:

  Y |
  G |
RGB | BBY
OWP | ORG
BPB | RGR
---------

The three B's in a row would be removed, two from the right player's side and one from the left.

Power blocks do things like remove all of a certain color of block from your or your partner's board, remove rows from you or your partner's board, add rows to your opponent's board, add stones (unremovable blocks) to your opponent's board, speed up your opponent's board, etc. The power blocks are stored up, in a FILO stack on the side of the play area.

A good site for information on Yahoo! Towers is: http://www.spacemoose.com/towers/faq.htm
Play at http://games.yahoo.com/games/login?page=yt