I've used every major gesture drawing application available and want to give a useful comparison for people choosing between them.

Line of Action: my primary tool. It has separate modes for figures, hands, animals, faces, and environments. The timer is configurable. The pose quality is high and varied. Free to use.

Quickposes: similar functionality to Line of Action. The interface is slightly cleaner. Has a good selection of clothed figures, which is useful for character design practice. Free with paid premium tier.

Sketchy (formerly SenshiStock): uses creative commons-licensed photographs. Better for study in realistic contexts than stylized gesture work.

PoseMyArt: lets you set up custom lighting and scenes with 3D models. Useful for specific composition or lighting challenges you can't find in photo collections. Paid subscription.

My recommendation: Line of Action for daily gesture practice, Pose My Art for specific compositional reference needs. The other tools are reasonable but don't add enough unique value to justify maintaining accounts in both.