| xref | IBM AIX Cross Reference Data |
| xresult | Xcode Xresult Data |
| xrf | Cal3D XML Materials Data |
| xrf | IMVU Material File |
| xrf | WordSearch Cross Reference Data |
| xrff | Weka eXtensible Attribute-Relation File Format Data |
| xri | XML for Requirements Interchange Data |
| xrio | OMICRON Test Universe Extended RIO Object |
| xrk | AiM Tech EVO4S Logger Data |
| xrl | Concordance Markup Data |
| xrl | OSCAR Rules Data |
| xrl | Rapidform XOR 3D Scan CAD Data |
| xrl | Sybase PowerDesigner Report Language Data |
| xrm-ms | Windows Vista Activation Certificate |
| xrnt | Renoise DSP Device-chains Data |
| xrp | Experior Practice Management Report Data |
| xrr | Chiron NRG Runtime Report Data |
| xrt | Revolution Single-spectrum Xraytor Data |
| xry | XRY Data |
| xrz | AiM Tech EVO4S Logger Data |
| xs | Hummingbird Exceed Startup File |
| xs8 | SIMUL8 XML Simulation |
| xsb | Sybase PowerDesigner XML Model Backup |
| xsb | XMLBeans XML Schema Binary |
| xsbz | Oracle BI Publisher Sub Template |
| xsc | RFM Molecule Cross-Section Data |
| xsd | ICQ 6 XSD Data |
| xsd | MAPublisher Custom Coordinate System Data |
| xsdif | Altova DiffDog Schema Data |
| xse | EQuIS Data Processor Format Definition Data |
| xseq | xLights Sequence Data |
| xsf | Cal3D XML Skeleton Data |
| xsf | IMVU Skeleton Data |
| xsf | Xilinx CORE Generator Pin File |
| xsi | Softimage dotXSI 3D Model |
