VLLasku
Free web-based invoicing primarily for Finnish companies
... and Finvoice electronic invoices. The code is written and commented in English, and invoices can be created in English. Multilanguage support/English UI is introduced in version 1.6.0. Some localization work will probably be required for other than Finnish companies.
N.B. Releases and source code are in a new location starting from version 1.6.0. See the home page for more information.