Opinions Wanted -- Tipping?

[kybearfuzz]

I'm usually a good tipper when it comes to restaurants unless the service is just horrible. However, when it comes to other services, I have no idea how much is an appropriate tip.

What do you consider to be an appropriate tip for the following?

1. A taxi ride (use $12 as an fare amount if you want)?
   
2. A haircut?
   
3. A hotel maid service?

Are there instances, like the maid service, where you don't feel you should leave a tip, or do you leave one if the room is necessarily messy?

Just curious...