The best way I have seen to retire now with only $100K to invest is to use the $100K to put 30% down on 7 rental properties purchased for $45K total each that are netting $1K/month each or $7K/month total after taxes, management and insurance, and $6K total per month after financing the remaining 70% @ 6% interest! These properties can be readily found in certain markets in the US such as St Louis, Kansas City, Cleveland, Indianapolis, Columbus, etc.
Sounds too good to be true, but the numbers don't lie and the properties are easier to find than you think!