If you start a new blank Windows 8 App in Visual Studio you get loads of app bar button styles to choose from in StandardStyles.xaml including:

SkipBackAppBarButtonStyle, SkipAheadAppBarButtonStyle, PlayAppBarButtonStyle, PauseAppBarButtonStyle, EditAppBarButtonStyle, SaveAppBarButtonStyle, DeleteAppBarButtonStyle, DiscardAppBarButtonStyle, RemoveAppBarButtonStyle, AddAppBarButtonStyle, NoAppBarButtonStyle, YesAppBarButtonStyle, MoreAppBarButtonStyle, RedoAppBarButtonStyle, UndoAppBarButtonStyle, HomeAppBarButtonStyle, OutAppBarButtonStyle, NextAppBarButtonStyle, PreviousAppBarButtonStyle, FavouriteAppBarButtonStyle, PhotoAppBarButtonStyle, SettingsAppBarButtonStyle, VideoAppBarButtonStyle, RefreshAppBarButtonStyle, DownloadAppBarButtonStyle, MailAppBarButtonStyle, SearchAppBarButtonStyle, HelpAppBarButtonStyle, UploadAppBarButtonStyle.

If that list doesn't quite cover you then it's really easy to create a metro button style that will. The easiest way is to create a style based on AppBarButton and to use an icon from the font Segoe UI Symbol. The advantage of using a font is that it is automatically scaleable. Segoe UI Symbol has a lot of symbols for you to choose from and is the font used by AppBarButtonStyle, which we are basing our style on. You can use Character Map to find a good one, or I've included some of my favourites at the end of this post.

Here is my style for a crop button. Include this style in a resource dictionary that will be accessible to all the xaml files that need it:

<Style x:Key="CropAppBarButtonStyle" TargetType="Button" BasedOn="{StaticResource AppBarButtonStyle}">
 <Setter Property="AutomationProperties.AutomationId" Value="CropAppBarButton"/>
 <Setter Property="AutomationProperties.Name" Value="Crop"/>
 <Setter Property="Content" Value="&#x25F0;"/>
 </Style>

The text in bold is the text you need to change to make different sorts of buttons. The Content property contains the hex unicode value for the font character you want to use as your icon. I wasn't aware of an accepted standard for a crop icon, so I found one that I thought worked pretty well.

To use this style on your button you just x like this:

<Button x:Name="Crop" Style="{StaticResource CropAppBarButtonStyle}" Click="OnCropButtonClick"/>

Here is a table including most of the Segoe UI Symbols that you're likely to find useful: