[c00lio]

There was taxbird for filing (there is another application with the same name that does something different, you want(ed) https://de.wikipedia.org/wiki/Taxbird ). But that has been killed by the tax authorities: They shut down the interface used there, so nowadays only proprietary applications exist.

But from experience, don't bother with any software, German taxes are sufficiently weird that only the most trivial cases will really work out, even if you pay lots of money for it. Get a tax accountant. It isn't just putting numbers into forms, it is more the problem of structuring your life around possible tax savings. Software won't help with that.

If you only ever need filing and putting numbers into forms, you can file your taxes on the official website: https://www.elster.de . You'd need a login anyways to authorize your tax accountant or enable your software to submit its data.