Revman (from the Cochrane Collaboration) - for protocol documentation, either linked to Cochrane or standalone application on a personal computer
Rayyan - primarily supports title and abstract screening; free; easy to use but relatively simple functionality
Abstrackr - only useful for screening; free; easy to use but limited functionality
Subscription or Purchase Models
DistillerSR - citation management to data analysis; subscription-based (moderate to expensive, discounts for Cochrane reviews); supports all kinds of systematic reviews; a bit more complex to use but excellent help support
Covidence - citation management to data analysis; subscription-based (relatively inexpensive; free for Cochrane reviews); designed for systematic reviews of interventions based on RCTs; easy to use