<p>If I've got a variable that contains the fully-qualified name of a file (for example, a project file), should it be called <code>projectFile</code>, <code>projectFileName</code> or <code>projectPath</code>? Or something else?</p>

I usually go with these:
```
FileName
```
for the file name only (without path)
```
FilePath
```
for the parent path only (without the file name)
```
FileFullName
```
for the fully qualified name with path
I don't think there is such a thing as an accepted standard for this. I depends on your (team's) preferences and whether you need to differentiate between the three in a given situation.
EDIT: My thoughts on these particular naming conventions are these:
intuitively, a "Name" is a string, so is a "Path" (and a "FileName")
a "Name" is relative unless it is a "FullName"
related variable names should begin with the same prefix ("File" + ...), I think this improves readability
concretions/properties are right-branching: "File" -> "FileName"
specializations are left-branching: "FileName" -> "ProjectFileName" (or "ProjectFileFullName")
a "File" is an object/handle representing a physical object, so "ProjectFile" cannot be a string
I cannot always stick to these conventions, but I try to. If I decided to use a particular naming pattern I am consistent even if it means that I have to write more descriptive (= longer) variable names. Code is more often read than written, so the little extra typing doesn't bother me too much.