We have been using Insperity and all its apps (including ExpensAble, and TimeStar). It's a full white-glove service that’s certified and well-respected, though it’s expensive and forces you to comply with regulations early.
Slack is great. It has helped reduce some of our overhead emails, and it makes it easy to have internal discussions and share knowledge. This is especially helpful since we have offices and partners all over the world, and many remote employees.
We love Confluence. It's helped take us out of email, and we also use it, together with Kaltura MediaSpace, for collaboration and knowledge management across the company.