Transparent labels
Matlab labels can be set to a transparent background as well as padding.
Optical illusion
Auto-completion widget
Matlab includes a variety of undocumented internal controls that can be used for an auto-completion component.
Plot legend title
Titles to plot legends are easy to achieve in HG1 (R2014a or earlier), but much more difficult in HG2 (R2014b or newer).
Frameless (undecorated) figure windows
Matlab figure windows can be made undecorated (borderless, title-less).
Unorthodox checkbox usage
There are various ways to display interactive split-panes in Matlab GUI. Uiextras flex-panels override a checkbox control's CData to display a divider.
Transparency in uicontrols
Matlab uicontrols' CData property can be customized to provide background transparency.
HG2 update
HG2 appears to be nearing release. It is now a stable mature system.
uiinspect
uiinspect is a Matlab utility that displays detailed information about an object's methods, properties and callbacks in a single GUI window.
Pinning annotations to graphs
Annotation object can be programmatically set at, and pinned-to, plot axes data points.