gravy train

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang An occupation or other source of income that requires little effort while yielding considerable profit.

Wiktionary

1. n. An occupation or any lucrative endeavor that generates considerable income whilst requiring little effort and carrying little risk.
2. n. A gorging on luxuries, since someone else foots the bill.

WordNet 3.0

1. n. income obtained with a minimum of effort