I looked at them, and was confused by the difference between their "cloud compute" offering and what I think of as a traditional linux VPS. But after a bit more investigation, it seems that this "cloud" stuff is the way things are charged now.

So the ones I've looked at:

Rackspace: carbon neutral via tree-planting, inexpensive, lowest-end VPS around $10/month, 4GB server from $175/month plus bandwidth.

Greenqloud: carbon neutral; uses 100% renewable energy (geothermal and hydro). Pricing almost identical to Rackspace.

AISO: carbon neutral, solar powered; pricing is about 3x Rackspace or Greenqloud.

Greenqloud is looking pretty good.