This is a workflow which lets you look up how much you have left in a You Need a Budget category for the current month.
It looks for your budget file in ~/Dropbox/YNAB and ~/Documents/YNAB. If you have a single budget in one of those folders, it will use that and needs no configuration. If you have multiple budgets, or a budget somewhere else, you can use a File Action on it to select it as the budget to use.
Fuzzy/substring matching, tries to use the currency symbol/format based on the budget’s configuration.
Hacked up the code to parse the YNAB data myself, so please make sure that it’s reporting accurately for you :)
Download here: http://cloud.jamesoff.net/2C04451d3R3Q
(Updated 2014-08-27 to fix 0.00 values being shown as “overspent”, and to add caching to speed up results.)