I've made an Open Source stop-watch application for tracking multiple JIRA issues at a time. It's simple but quite convenient. I would like some more eyes on it and some feedback from other peoples perspective.
For me, the use-case is: At work I'm usually working on up to 5 issues at a time - sometimes an issue is waiting for input from somebody temporarily away. Sometimes I need to prioritise a new issue above others. So I need to be able to quickly switch between issues - and still make sure to track how long I've totally worked on each issue.
So if you're interested, take it for a spin, and let me know if you have any suggestions for improving it. 
https://github.com/carstengehling/jirastopwatch
It's Windows-only, full source available, and also a convenient setup-file. The application uses Jira's REST API v. 2, so your version of JIRA must support this.
It would be interesting for me to know, if it also works with JIRA Cloud.