categoriesByPayees := Dictionary new.
FileStream readOnlyFileNamed: 'payees-by-categories.txt' do: [ :file |
    " With #readOnlyFileNamed:do: you don't have to worry about closing the file,
    it ensures that #close is sent to the file, even if you leave the block through an Exception. "
    [ file atEnd ] whileFalse: [
        | parts |
        parts := (file nextLine findTokens: $| escapedBy: Character tab)
            collect: #withBlanksCondensed.
        " We collect the parts #withBlanksCondensed, so we don't have to call it repeatedly in the loop.
        You can use unary selectors in place of blocks with one argument. You may look at the implementation
        of Collection >> #collect: and Symbol >> #value: to find out why. I'm not sure what you really want is
        #findTokens:escapedBy:, it cuts the string along | characters if they are not preceded by a tab."
        parts size > 1 ifTrue: [ 
            " We skip blank lines or categories without at least one payee. "
            | category |
            category := parts first.
            parts allButFirstDo: [ :payee |
                (categoriesByPayees
                    at: payee
                    ifAbsentPut: [ OrderedCollection new ]) add: category
                    " Your implementation only remembers the last category encountered for a payee.
                    We can collect all the categories. " ] ] ] ]​

Cheers, Balázs