Well, lets see...
For Celtic music (this is a very small sampling of my Celtic tastes):
Solas - Go for their self-titled album.
Lunasa - Same as above. I like their first and self-titled album.
Kate Rusby - *LOVE* her. 'Sleepless' is a ncie place to start.
I love Arabic Pop - Natacha Atlas(Egyptian) is a good artist to start with, I recommend the album 'Gedida'. I also recommend Nawal Al Zoughby, a Lebanese pop singer.
Also love Indian music - Ravi Shankar (of course), but for a little bit of a twist, try DJ Cheb I Sabbah. I find that 'Shri Durga' is a great CD to accompany my Vinyasa Yoga practice.
If you like Buena Vista Social Club, it's safe to say that most of Ry Cooder's world music projects will appeal to you. I particulary like, 'A Meeting by the River', with Mohan Vishwa Bhatt.
For African music, I have to say that 'Talking Timbuktu' by Ali Farka Toure is a must-have. And of course, Ladysmith Black Mambazo.
French? Francoise Hardy, for sure. And Edith Piaf, but she's an aquired taste.
Bulgarian music is amazing. 'Les Mystere Des Voix Bulgares' is an oldie but a goodie. Quite haunting. Bulgarian Wedding Music is *SO* much fun. I saw the Superstars of Bulgarian Wedding Music reunion at the International Folk Festival last year, and was completely blown away. 'Together Again' is their recent reunion album.
Philip Glass has collaborated with Uakti - a Brazilian ensemble to produce a brilliant album, 'Aguas da Amazonia'. It's wonderful.
And of course, love those Gypsy Kings!
Hope this sampling is some sort of helpful - if not random- foundation for an expanded World Music collection.