Selecting

Box Select
Click and drag to box select multiple nodes. Alternatively, B starts the bounding box selection process as well.
Lasso Select
Ctrl-Alt-LMB click and drag starts a lasso selection.
(De)select All
A
Inverse
Ctrl-I
Select Linked From
L
Select Linked To
Shift-L
Select Grouped Shift-G

Selects similar nodes to the active node by their properties.

Type
The node type. e.g. all Math nodes.
Color
The color property.
Prefix, Suffix
Matches the name property from start/end of the text.
Activate Same Type Previous/Next Shift-]/Shift-[
Finds the previous/next node of same type, activates the node, and ensures the node is visible.
Find Node Ctrl-F
To search for a node. On selecting a node, it activates the node and makes sure the node is visible.
Select multiple
Shift-LMB or Shift-RMB used for multiple node selection.