Other
The Sigma Notation for Summation of Series
Conprehensive site explaining sigma notation. Site has many formulas and a few examples
Math Words
Math words.com: Sigma Notation
MathWords.com offers a very brief definition for sigma notation along with an example which illustrates what it is.
Purple Math
Purplemath: Sequences and Series: Terminology and Notation
Explains the basic terminology and notation of sequences and series, including summation symbols, subscripts, and indices.