Interesting... Seems like wesabe has a lot of interesting information at their disposal but I keep having trouble thinking of a concrete and useful task for it. There must be some good ideas there though.
I've tried to use tools like GnuCash and Buxfer to keep track of every dollar I spend and receive. Seemed like a good idea, but I've decided that my personal finances aren't complex enough to warrant frequent checking.
The only features I want with Buxfer/Wesabe is to be able to check my balance over SMS. I don't want to enter individual transactions or manually upload QIF files from my bank's website so it should be imported automagically. Periodic reminders to pay the rent might be useful too.
Fortunately, Wesabe has a tool that retrieves your bank data and uploads it to their servers though they lack the SMS features and their site is fugly. Buxfer looks pretty good and has the SMS features, but I have to submit my bank data through a web form.
If Buxfer implemented what Wesabe has, I'd definitely use it. Maybe I'll just write something to grab my bank data and upload it to Buxfer in the meantime.
I don't think personal finance is that interesting in terms of the api. Maybe small business could use the API to integrate with something tracking their funding, but there are already traditional tools for such things.
I wonder if you can - just from the description of each transaction - decide what company is getting your money. If you know who is getting it, you can correlate that a database of the companies country of origin, type of business, location, etc. Then you can imagine a set of mashups you can do.
There might be some collaborative filtering that can be done as well, based on your purchases and perhaps rating of satisfaction of those purchases... This would require people to share (even if anonymously) their spending patterns.
