two columns with borders

Just float one element - the disadvantage is that the unpositioned div is actually behind the floated div:

Look! No scroll bars! (floated right)
Look! No scroll bars! (No positioning)

Or use wrapper divs - may sometimes wrap in IE5.x due to pixel rounding errors:

Look! No scroll bars! Floated left
Look! No scroll bars! Floated left

Fix the pixel rounding errors by making one of the divs 49.9% wide:

Look! No scroll bars! Floated left
Look! No scroll bars! Floated right, 49% wide

Absolute positioning one at left:0, the other at left:50%:

Look! No scroll bars! left:50%
Look! No scroll bars! left:0