Not exactly. You can't create custom fields, but we do offer Custom Statuses and fields for Category and Milestone that you can customize. In our experience custom fields end up only being used for a fraction of the time. For instance, if you add a custom field for user agent or URL, you end up with a field that's not applicable when someone is simply logging a project issue and not a bug. Similarly, even when entering a bug, URL and user agent are only relevant in certain situations. Our philosophy is that it's best to keep the fields to the attributes that are applicable regardless of what kind issue you're capturing. Everything else can be placed in the description and still be available for searching via Sifter's full-text search.