ShaderLab builtin values
Unity provides a handful of builtin values for your shaders: things like current object's transformation matrices, time etc.
You just use them in ShaderLab like you'd use any other property, the only difference is that you don't have to declare it somewhere  they are "built in".
Using them in programmable shaders requires including UnityCG.cginc file.
Transformations
 float4x4 UNITY_MATRIX_MVP
 Current model*view*projection matrix
 float4x4 UNITY_MATRIX_MV
 Current model*view matrix
 float4x4 UNITY_MATRIX_P
 Current projection matrix
 float4x4 UNITY_MATRIX_T_MV
 Transpose of model*view matrix
 float4x4 UNITY_MATRIX_IT_MV
 Inverse transpose of model*view matrix
 float4x4 UNITY_MATRIX_TEXTURE0 to UNITY_MATRIX_TEXTURE3
 Texture transformation matrices
 float4x4 _Object2World
 Current model matrix
 float4x4 _World2Object
 Inverse of current world matrix
 float3 _WorldSpaceCameraPos
 World space position of the camera
 float4 unity_Scale

xyz
components unused;.w
contains scale for uniformly scaled objects.
Lighting
In plain ShaderLab, you access the following properties by appending zero at the end: e.g. the light's model*light color is _ModelLightColor0
. In Cg shaders, they are exposed as arrays with a single element, so the same in Cg is _ModelLightColor[0]
.
Name  Type  Value 
_ModelLightColor  float4  Material's Main * Light color 
_SpecularLightColor  float4  Material's Specular * Light color 
_ObjectSpaceLightPos  float4  Light's position in object space. w component is 0 for directional lights, 1 for other lights 
_Light2World  float4x4  Light to World space matrix 
_World2Light  float4x4  World to Light space matrix 
_Object2Light  float4x4  Object to Light space matrix 
Various
 float4 _Time : Time (t/20, t, t*2, t*3), use to animate things inside the shaders
 float4 _SinTime : Sine of time: (t/8, t/4, t/2, t)
 float4 _CosTime : Cosine of time: (t/8, t/4, t/2, t)
 float4 _ProjectionParams :
x
is 1.0 or 1.0, negative if currently rendering with a flipped projection matrix
y
is camera's near plane
z
is camera's far plane
w
is 1/FarPlane.  float4 _ScreenParams :
x
is current render target width in pixels
y
is current render target height in pixels
z
is 1.0 + 1.0/width
w
is 1.0 + 1.0/height