Just to add some thoughts:
Google is also in the SaaS space with GMail, GoogleDocs, and until
recently Google Video.
Microsoft will likely also start to show up in the SaaS spaces over
2009-10, given the nature of their strategy (that's my speculation at
least)
SalesForce.com operates on Force.com, and that is a PaaS offering that
is available as well.
IBM and Sun both play in the IaaS space today, and I thought I had
heard that HP and AT&T might be moving into this area soon too.
I'd personally have a hard time calling out "big players" in SaaS,
mainly because as this area matures the "big" players will be the apps
that people need and can operate in the Cloud well. Over time this
will be rated more like apps today with "big" being relative to the
functionality of the app. For example, office / knowledge work
provider, CRM provider, security services provider, etc. Once you
break out to providing multiple disparate functions in the Cloud I'd
think we begin to see them move from SaaS to PaaS.
Big PaaS today - Force.com, GoogleApps. Soon to be Microsoft (Azure,
LiveMesh, S + S)
Big IaaS today - Amazon, IBM, Sun. Soon to be? HP and AT&T
I wholeheartedly agree that mix and match would be great and very
necessary for the Cloud Ecosystem to really reach the overall promise,
but the technology cynic in me thinks that no vendor will be motivated
to create open and interoperable standards from the start, rather
doing the traditional "try to get a big market share then drive the
standard to what we are doing" model. I hope I'm wrong on that. :)
The good news on this front comes from the Open Source area IMO, which
will drive adoption of better standards faster because to get the
mobility and agility required, without finding themselves locked to a
single vendor, the big enterprise customers will have to insist on
interoperabilty so they can purchase best of breed vs single vendor.