After a bit of a think I realised that this question can also be asked the other way round and much more simply: In what situation(s) have you required a model release?
Questions about model releases come up fairly often so I'd like to make a community Wiki about it. When answering please make sure to specify that you are not a lawyer and the legal locale (i.e. Country, and State if necessary).
Which of the following scenarios legally requires a model release and what terms does the release need to contain?
Public A: Aphotographer takes a photo in public place such as a park or street. The photo includes identifiable people, and the photographer wants to use the photo in their own portfolio or website
Public B: Same scenario as A but end use is a non commercial publication or website
Public C: Same scenario as A but end use is a third party commercial publication, website, or stock library
Public D (trick question): A photographer takes a photo in a shopping mall, theatre, stadium, museum, railway station, event or other privately-owned publicly-accessible location.
Model E: A photographer has a photoshoot with a model and gives the model prints or a CD of images and wants to use the photo in their own portfolio or website
Model F: Same scenario as E but end use is a third party commercial website
Model G: Same scenario as E but end use is an 'erotic' site
Model H: A photographer does a photoshoot with a model and pays for their time with money at an agreed rate and wants unlimited use of the photo commercially
Model I: A photographer takes some photos of a friend at the friends request and wants to use the photos in the photographers portfolio/website
Model J: A photographer takes photos of their own children and wants to use the photos in their portfolio/website
Model K: As scenario J but for commercial use
Event L: A photographer is paid to take photos of everyone at a wedding or other event on private land and wants to use the photos on their portfolio/website
Event M: As scenario L but end use is a commercial publication or website
Event N: A photographer takes photos of a fireworks display in a public park and wants to use the photos commercially
I think I've covered most of the situations photographers will find themselves in but I'll add in any suggestions. Note again that this is intended for community wiki and that locale is important in answers.