Kathy bags always have super cute charms, coach also had accessory charms to buy separately. These are some cute ones also ... http://******/bZCrGG from Hayden-Harnett (NYC).
I have two Prada robot charms (a silver saffiano one and on called 'hook'), a giant rooster one and a heart shaped one by Mulberry, an orange Hermes Carmen, and a mini boogie bag by Celine. Although I love all of mine I hardly every put them on my bags as they are all quite large and some are heavy and thus can be a bit of a hassle on my bags.
A lot of people use charms from Juicy Couture, Tarina Tarantino, Coach,Louis Vuitton. I use keychains too. It's really just about using your creativity. I know Etsy has wonderful items there!
I love Tarina Tarantino charms. The website only has a Hello Kitty one, but the NYC store has hearts (both crystal and leather), bows, etc. They start around $60.