Can I use cassandra with heroku?

No and yes.

It depends on whether you mean use a Heroku-hosted Cassandra instance or not. If that's what you mean then the answer is no (for now at least). They support PostrgeSQL within their stack and MySQL through Amazon RDS. MongoDB support is in beta (partnering with MongoHQ) and CouchDB is roadmapped for later this year (in conjunction with Cloudant).

But of course there's no technical reason to stop you from deploying Cassandra elsewhere (say on EC2) to work with your Heroku app. So in this context the answer is yes.

UPDATE (2012/09):

Heroku has launched a new add-on for Cassandra hosted by Cassandra.io. Details available here.

UPDATE (2014/11):

The Heroku Cassandra beta addon is no longer available nor does cassandra.io seem to exist anymore. ATM no Cassandra addon is available so only rolling your own as detailed above is possible.

Yes take a look at http://cassandra.io for a hosted Cassandra solution and heroku addon.

We are in private beta as you can see here: https://addons.heroku.com/cassandraio

You can provision the addon using the following command:

heroku addons:add cassandraio