Donating is the easiest and fastest way to efficiently support KDE and its projects. KDE projects are available for free therefore your donation is needed to cover things that require money like servers, contributor meetings, etc.
KDE e.V. is the non-profit organization behind the KDE community. It is based in Germany and thus donations are tax deductible in Germany. If you live in another EU country your donation might also be tax deductible. Please consult your tax advisor for details.