How it gets its data. Three mechanisms, and they aren't interchangeable. Streak and NetHunt render inside Gmail, so the thread you're looking at is the record. Salesflare mines the mail: it gathers contact and company details from emails, email signatures and social profiles, and logs your meetings and calls. Happenstance reads only the envelope, who you wrote to and how often, and uses that to rank people rather than to file messages.
What it reads. A CRM that logs and tracks messages reads the messages. Happenstance reads Gmail as headers only: it tells you who you know and how well, and message content never reaches its servers.
Free plan or trial. One permanent no-cost tier among the paid set, HubSpot's, capped at 1,000 people and two users with no time limit. Streak, Copper, NetHunt, Salesflare and folk offer nothing unpaid. Trials run 14 days at NetHunt and 30 at Salesflare.
What the annual rate hides. Every vendor here quotes the annual number. Month to month, NetHunt Basic is $30 rather than $24, Salesflare Growth is $39 rather than $29 and Streak Pro is $59 rather than $49. If you won't commit to a year, budget from the monthly column.