A list of pages on non default apps, for an index of default apps, see Default Applications.
Thunar file manager (note: this is a default app in Statler)
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sylpheed e-mail client. It's lighter weight than Claws which is a fork that's added many other features. You can also add features with slypheed, but it's taken the approach of shelling out to other programs for added functionality similar to classic Unix philosophy of do one thing well. It has better security, since it limits scripts that can be run and tries to keep to text based viewing. It works better cross-platform, so you can use the same e-mail client on multiple operating systems.