A bushing is a plain bearing (no rolling elements) that only provides a bearing surface. They are less noisy than linear ball bearing but not always practical to integrate.
|Goldmine||US||Bushing with flange||8 mm||bronze||Don't use on 5/16" rod.|
|Paoparts||FR||Iglidur D6||6 mm||plastic|
|Paoparts||FR||Iglidur D8||8 mm||plastic|
|TechPaladin||US||Sleeve Bushing||8 mm||bronze||Self-aligning.|
One RepRapper has used Ultra High Molecular Weight Polyethylene to create self-lubricating bushings that are less noisy, but deform under load and have increated wear and play over time compared to linear bearings. More information in DIY linear bearings (no PLA) on the forum.