From Extended Resizer & SplitBar:
SplitBar XT adds horizontal / vertical split bars and multiple window panes into your application without any code. Individually adjust the size and position of each control based on rules you set. SplitBar XT handles hosting, form resizing and splitting in a smart and effective way thus providing full customization using advanced properties and events. Fast to implement with WYSIWYG concept. Design professional looking user interface in no time.