I relied on much cruder predecessors of this app back when I was a Web designer. QuickLens offers the ability to:
- Zoom into pixels
- Sample colors and export as Html/CSS color sheet or Photoshop Swatch
- Measure dimensions and angles
- Inspect alignments with screen-wide guides
- Overlay grids
- Take snapshots
- Simulate vision defects
The app is $14.99 on the Mac App Store.