Some of my favorites:
Ballistic Fabrication - had shipping problems for a while but they have cleaned that up. Great components and supplies.
Currie - I love Johnny Joints except when I have to disassemble them. Always great products and lots of options to play around with.
JcrOffroad - a new one for me, ordered a tapered reamer from them, somehow it got lost in shipment. Sent them an email about it this morning, got an email from them within an hour, they're sending me another one and apologized for the delay. Really nice people to deal with.
Rokmen - what can I say, if I can't make it I want their stuff. Incredibly good people and they build the most awesome stuff...