Languages: Java backend, AS3 frontend, Python + Ruby misc
Note that we're very new (and hiring!), so much of this is still being built out.
Software lifecycle: git for SCM (GitHub as central repo); ant for builds; ivy for dependency management; Bamboo for build automation; Crucible for code review; JIRA for issue tracking; Greenhopper for sprint planning; Vagrant for containerized development environments
Systems infrastructure: AWS (EC2 w/ELB+ASG and S3 for now, more services to come); nginx-fronted Tomcat for JVM; Cassandra for persistent DB; chef for configuration management (currently chef-solo, but evaluating Opscode Platform); PoolParty+Fabric for one-click deploy server fleets; Nagios + Pingdom for monitoring; Mixpanel + Kontagent for analytics; Dynect (likely) for DNS, possibly for GSLB; Akamai (likely) for CDN, possibly for GSLB
Misc: PagerDuty for on-call stuff; Google Apps for email, shared docs, etc; Confluence for wiki; OpenFire (Jabber) for chat