Yes, both Userback and Sentry offer a Free Trial.
Userback is designed for Individuals, Freelancers, Medium Business and Small Business.
Sentry is designed for Large Enterprises, Medium Business and Small Business.
The starting price of Userback begins at $14/Month when Billed Yearly, while pricing details for Sentry are unavailable.
Some top alternatives to Userback includes Backlogs, Backlog, ReportBugz, Buglog, Usersnap, Ybug, Bug-Track.com, Disbug, Feedback Monkey and Toybox.
Some top alternatives to Sentry includes The Bug Squasher, Bugsnag, Zoho BugTracker, Rollbar, TrackJS, BugHost, Bug-Track.com, Disbug, Hawk and Crash Catch.