Even though knowing this developmental reality may not remove your aggravation, you should be heartened that her oppositional behavior (yes, even her throwing herself on the floor) is a healthy part of her growth. She doesn't have the language skills to match her intense emotions and this causes her to act in these dramatic ways.
During these moments when she does not want to hear "No" from you, her contrary reactions are not really manipulative, she is genuinely overwhelmed by her feelings and falls apart. Please try not to ascribe ADHD to her or consider giving her drugs -- she is a normal two year-old, and you are first-time parents are searching for explanations for your "sweet, happy little girl's" explosive outbursts at being told "no" or being disciplined.
Your child needs to know rules, limits, and boundaries. Her testing these out is a way for her to grow and understand the reasons behind these guidelines and restrictions. She can learn them without being punished or rewarded, spanked or shamed. She can be given more choices in her everyday life to make her feel more powerful and independent -- even asking her what color pants she wants to wear will make her feel more in charge.
Tantrums can often be headed off by paying attention to the cues that she gives right before she would normally throw herself on the floor. Respond to these signals with empathy and redirect her attention and energy.
Here are three fine books that will give you practical tips for parenting your toddler and for disciplining her with love:
Setting Limits: How to Raise Responsible, Independent Children by Providing CLEAR Boundaries by Robert J. MacKenzie
First Aid for Tantrums by Kathy Levinson
Parenting your Toddler by Patricia Henderson Shimm