addict, freak, junkie, junky, nut - someone who is so ardently devoted to something that it resembles an addiction; "a golf addict"; "a car nut"; "a news junkie"

backslapper - someone who demonstrates enthusiastic or excessive cordiality

balletomane - a ballet enthusiast

fanatic, fiend - a person motivated by irrational enthusiasm (as for a cause); "A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill

gadgeteer - a person who delights in designing or building or using gadgets

shutterbug - a photography enthusiast

sports fan, fan - an enthusiastic devotee of sports

admirer, booster, protagonist, supporter, champion, friend - a person who backs a politician or a team etc.; "all their supporters came out for the game"; "they are friends of the library"

adorer, admirer - someone who admires a young woman; "she had many admirers"

animal fancier - a person who breeds animals

bird fancier - a person have a strong interest in birds

technophile - a person who is enthusiastic about new technology