According to The Complete Idiot's Guide to Etiquette…
Babysitter or nanny: One week's pay plus a small gift
Trash collectors: $15–$20 each
Paper carrier: $15–$25 for daily delivery and $5–$15 for weekend delivery
Mail carrier: As federal employees, mail carriers can't accept more than $20, so $15–$20 is fine.
Doorman: $25–$120, depending on if he works with staff or alone.
Hairdresser: The equivalent of one session (or $25 if you're not a regular salon-goer)
If you're deciding between money and a present, it might help to know most people prefer the cash.