[ale] VOIP recommendations?

[Brian W. Neu]

I'm looking to recommend to several of my clients that they ditch the
stone ages and leap into the world of today with some VOIP services. 
They are going to want desktop phones though -- possibly travel-friendly
phones.

Can I get recommendations from my open source amigos for hardware and
service providers?  I'm not a stickler for some kind of Linux solution,
just a solid one.