Suggestions on tracking staff time against a contract?

I'm looking for any suggestions on the best way (either via Salesforce or a third party app, as long as it's free for nonprofits!) for program staff to keep track of their time in particular projects, so we can compare hours worked to the contracted commitment. The goal is AS user-friendly as possible, as program staff are not super tech-savvy or familiar with Salesforce. I found a couple apps on the appexchange, but would love to hear about others' experience!

Thank you!