Blocksum (Windows, freeware, 32MB) - This is one game that will destroy your social life. Join numbered blocks together to add their amounts and cause blocks with the same digits to vanish. As more bricks rise from the bottom of the screen, you must frantically add them together to keep the stack from reaching the top. It's more puzzle gaming enjoyment than I've had in a very long time, and the pacing is perfect to keep you motivated but not frustrate you. Nice visuals and really catchy music are extra special bonuses.
NOTE: Some comments, from when this game was first mentioned on the site, may be found on this page.
Update