In the rippletrade client, I'd like to suggest these two new features:
Create a list of labels
In transaction history or at the payment/convert screen, assign label to transaction
This would allow people to mark their transactions as business, personal, or whatever other categories the user decides to include besides these two use cases.
Edit: I was thinking of this a bit more. If this could be a two-dimensional array called user_labels then user_labels could default to Parent Label and Child Label respectively (if left undefined by the user). This could allow for 'Business > Web Hosting Payment' labels or 'Personal > Payment to Johnny' labels.