APSARD is, for my money, the central professional organization in the US for clinicians and researchers interested in ADHD and perhaps my favorite annual professional conference to attend.
CHADD is the largest ADHD organization and it hosts a wonderful annual ADHD Conference, co-sponsored with the Attention Deficit Disorder Association and ADHD Coaches Organization
ADDA was the first ADHD organization focused specifically on adults and was instrumental in establishing October as National ADHD Awareness month.
The World Federation is a wonderful international ADHD organization that hosts biennial conferences.
The Canadian ADHD Resource Alliance is a font of very useful, scientifically based information and sponsors a high-quality annual conference with both a research day and general conference.
ABCT is the central organization related to evidence-supported cognitive-behavior therapy for numerous conditions.
Credible information provided in an entertaining fashion by actor/comedian/writer turned ADHD advocate, Rick Green.
UKAAN is a great source of information and support for adults with ADHD throughout the UK.
If by chance you want to learn more, here are means by which to follow or contact me.