There are many features and settings that I'm finding on the PlantUML Q&A and are not documented.
Also many don't appear when the -language parameter is used.
Here are some:
http://forum.plantuml.net/6010 skinparam arrowMessageAlign center
http://forum.plantuml.net/6105 Activity diagram break
http://forum.plantuml.net/5201 skinparam swimlaneWidth
http://forum.plantuml.net/6289 %filename% and %filenameNoExtension%
http://forum.plantuml.net/6199 %PLANTUML_VERSION%
http://forum.plantuml.net/3931 !pragma useVerticalIf on
http://forum.plantuml.net/1132 !pragma horizontalLineBetweenDifferentPackageAllowed
http://forum.plantuml.net/2335 allow_mixing
http://forum.plantuml.net/2335 mix_
http://forum.plantuml.net/1047 skinparam style strictuml
http://forum.plantuml.net/977 skinparam nodesep
http://forum.plantuml.net/977 skinparam ranksep
http://forum.plantuml.net/4411 skinparam colorArrowSeparationSpace
http://forum.plantuml.net/6495 skinparam tikzFont
http://forum.plantuml.net/3820 skinparam tabSize
http://forum.plantuml.net/3193 skinparam groupInheritance
http://forum.plantuml.net/4247 hide unlinked
http://forum.plantuml.net/2794 skinparam lifelineStrategy
http://forum.plantuml.net/1290 skinparam conditionStyle
http://forum.plantuml.net/5968 %filedate%
http://forum.plantuml.net/5493 skinparam ParticipantPadding
http://forum.plantuml.net/5493 skinparam BoxPadding
http://forum.plantuml.net/5849 Salt "sprite"
http://forum.plantuml.net/5840 Salt titled groupbox
http://forum.plantuml.net/5835 Notes on class fields
http://forum.plantuml.net/5826 backward on loops
http://forum.plantuml.net/5793 @pause and @unpause
http://forum.plantuml.net/5782 Arrow on Gantt Diagram
http://forum.plantuml.net/5776 Timing diagram coloring
http://forum.plantuml.net/5769 %filename% and %dirpath% variables
http://forum.plantuml.net/5769 @startdef and @enddef
http://forum.plantuml.net/5647 BPMN (@startbpm and @endbpm)
http://forum.plantuml.net/5510 Gantt diagram dates
http://forum.plantuml.net/5493 skinparam ParticipantPadding and skinparam BoxPadding
http://forum.plantuml.net/5483 @startuml filename for automatic extensions
http://forum.plantuml.net/5453 skinparam pathHoverColor
http://forum.plantuml.net/5340 inline coloring of borders
http://forum.plantuml.net/5261 crows foot edge
http://forum.plantuml.net/5254 <plain></plain>
http://forum.plantuml.net/5171 Boolean not (!) and parenthesis on !ifdef
http://forum.plantuml.net/5037 -pipemap parameter
http://forum.plantuml.net/1626 Activity diagram GOTO
http://forum.plantuml.net/4949 Inline setting for edge style and thickness
http://forum.plantuml.net/4906 Gradient color on activity diagram partition
http://forum.plantuml.net/4752 Generate Braille diagrams (-tbraille)
http://forum.plantuml.net/4637 skinparam minClassWidth on sequence diagram
http://forum.plantuml.net/4596 \startXXX instead of @startXXX
http://forum.plantuml.net/4470 Label of arrows of repeat while
http://forum.plantuml.net/4467 @startuml with identifier for includes
http://forum.plantuml.net/4418 !pragma svek_trace
http://forum.plantuml.net/1226 !pragma ratio
http://forum.plantuml.net/4309 State diagram <<expansionInput>> and <<expansionOutput>> , <<inputPin>> and <<outputPin>> , <<entrypoint>> and <<exitpoint>>
http://forum.plantuml.net/4266 -config parameter
http://forum.plantuml.net/4260 bold and plain arrow styles
http://forum.plantuml.net/4210 When using Dot, PlantUML supports strict graph and digraph
http://forum.plantuml.net/4181 More inline settings of arrows
http://forum.plantuml.net/4147 skinparam maxAsciiMessageLength
http://forum.plantuml.net/1159 State diagram <<choice>>, <<fork>>, <<join>>, <<end>>
http://forum.plantuml.net/4080 Import images from jar/zip
http://forum.plantuml.net/4076 and http://forum.plantuml.net/4079 Sprites on stereotypes
http://forum.plantuml.net/3931 !pragma useVerticalIf on
http://forum.plantuml.net/3868 Use tild (~) to escape @startuml
List continues on http://forum.plantuml.net/7140
