Kayako

Extensible - fits most of our odd configuration and workflow needs. Kayako allows us to act as multiple support-centers in order to support multiple clients (organizations). Each client is provided with client-care that is customized to them (i.e. response templates, inbound email addresses, outbound response addresses). Plus, I like the simple, yet powerful mechanism that allows us to distribute tickets to the proper employee.

UI could be updated a bit to something a bit more 'modern'. There is a lot of online 'help' that related to the older versions of the products which leads to a wild goose chase for information. We have had a lot of trouble figuring out how to create user-groups within the system, where each group is separate and distinct from each other group - in particular we seem to be unable to disable the online-visibility of one set of users from another in the system.

Kayako has a very robust set of configuration options - it might require a bit more set up to get going, but should be able to meet your demanding requirements. If you have really simple requirements this tool might be overkill - but should be considered regardless. We act as a support/call-center with multiple faces/personalities/brands to the public. The Kayako system was almost the only system that could be configured to provide those multiple interfaces.