Ok, it was a nice surprise (after writing it several times) to find that there already is a BooleanToVisibilityConverter in System.Windows.Controls namespace.
Probably there are more such hidden time-savers.
Anyone got some?
Answer
I did a quick trawl using the Object Browser and this is what I have.
Derived from IValueConverter:
System.Windows.Controls.AlternationConverter
System.Windows.Controls.BooleanToVisibilityConverter
System.Windows.Documents.ZoomPercentageConverter
System.Windows.Navigation.JournalEntryListConverter
Xceed.Wpf.DataGrid.Converters.CurrencyConverter
Xceed.Wpf.DataGrid.Converters.DateTimeToStringConverter
Xceed.Wpf.DataGrid.Converters.GreaterThanZeroConverter
Xceed.Wpf.DataGrid.Converters.IndexToOddConverter
Xceed.Wpf.DataGrid.Converters.IntAdditionConverter
Xceed.Wpf.DataGrid.Converters.InverseBooleanConverter
Xceed.Wpf.DataGrid.Converters.LevelToOpacityConverter
Xceed.Wpf.DataGrid.Converters.MultimodalResultConverter
Xceed.Wpf.DataGrid.Converters.NegativeDoubleConverter
Xceed.Wpf.DataGrid.Converters.NullToBooleanConverter
Xceed.Wpf.DataGrid.Converters.SourceDataConverter
Xceed.Wpf.DataGrid.Converters.StringFormatConverter
Xceed.Wpf.DataGrid.Converters.ThicknessConverter
Xceed.Wpf.DataGrid.Converters.TypeToBooleanConverter
Xceed.Wpf.DataGrid.Converters.TypeToVisibilityConverter
Xceed.Wpf.DataGrid.Converters.ValueToMaskedTextConverter
Derived from IMultiValueConverter:
System.Windows.Controls.BorderGapMaskConverter
System.Windows.Navigation.JournalEntryUnifiedViewConverter
System.Windows.Controls.MenuScrollingVisibilityConverter
Microsoft.Windows.Themes.ProgressBarBrushConverter
Microsoft.Windows.Themes.ProgressBarHighlightConverter
Note the Xceed ones (no connection) are available free with their DataGrid. As well as those there's some clever stuff around like the debugging converter. I've also used the last IValueConverter and I'm sure there's some further lambda function goodness to be found, too.