For Piwik 0.4.3 (and earlier): Click on the "lost password" link on the Login form. A new password will be sent to your registered email address.
For Piwik 0.4.4: Click on the "lost password" link on the Login form. A link to the Password Reset form will be sent to your registered email address.
If you don't have mail set-up, you'll have to edit either config/config.ini.php or the piwik_user table. Passwords are saved as an md5 hash.
Buy me a beer. Show me some love.
Changes in the upcoming release:
CHANGES