I never asked for donations, so there's no official button.
But if you really insist, you can check this 'hidden' page: www.hwinfo.com/contact_donate.html
EDIT: DONATE BUTTONS ARE NOW LOCATED AT THE BOTTOM OF THE HOMEPAGE
i decided to donate you 20€ because i like HWInfo very much and use it very often and it helped me a lot to optmize and analyse my system,
especially for fan control, cpu temperatures and optimizing fan speed curves in bios
thanks for this great tool
hope you will update HWinfo many years.
and your HWInfo makes my desktop look much greater