BKEY - Activates box-select tool.
BKEY BKEY - (Pre-Blender-2.5) Circle Select. Use NUM+ to increase, NUM- to decrease, or MW to adjust the circle size. Drag over vertices/edges/faces with LMB to add them to selection, MMB to remove them from selection. Leave Circle Select with RMB or ESC.
ALT+BKEY - Select portion of viewing area to only be visible.
SHIFT+BKEY - Render Border. This only works in Camera ViewMode. Draw a rectangle to render a smaller cut-out of the standard window frame. If the option DisplayButtons->Border is ON, a box is drawn with red and black lines.