Beside versus Besides

Beside (without the s) is a preposition that means next to, along side of, at the side of

• "The tree that is beside the running water is fresher and gives more fruit" ~ Saint Teresa
• The little girl was sitting beside her mother.
• A man should stand beside his woman.
• Who is the big girl sitting beside Anne?

Besides (with the s) can act as a preposition or a conjunctive adverb.

Besides means moreover, in addition, also, as well, in any case when it acts as a conjunctive adverb.

• Besides chocolate, you will also need a second flavoring in the ice-cream.
• I don't like those earrings; besides, they're t costly.
• I'm to exhausted to type the report tonight; besides, it's not due until next week.
• He is naive, immature and irritable; besides, he is too young for you.

Besides means except when it acts as a preposition.

• No one besides her mother knew her secret.
• No one besides the trainer could control the dog.
• Nobody besides Sam can deal with her
• No one besides Sam would use such an offending tone.

Idioms:

Beside oneself: very surprised; greatly agitated; in a state of extreme, uncontrolled emotion

• He was beside himself with anger.

Beside the point: not relevant or important

• Your idea is beside the point.