That's just what I was going to post. Allegra is a beautiful name itself, but I find it unusable today because of the popular allergy medicine. Allegria takes care of that.
This is my absolute favorite name ever. I love the fact that it means "lively". I will definitely use this someday, unless it gains too much popularity.
I prefer the spelling Allegra. Allegria looks more like 'allergy' than Allegra to me. Besides, I didn't hear of the allergy medicine until I saw it mentioned in the comments for Allegra.