I've not tried it yet, but some of my colleagues swear by the Jambox (by Jawbone) as something that works not only as a speaker but also as a speakerphone. It connects via bluetooth.
I'll probably borrow one for my next call and see if it works as well as they claim ;-)
Thanks. I found a simple, unpowered (believe it or not, made from bamboo) made in the UK that works unbelievably well. Price was around $40 US.