| xch | Link Assistant Project |
| xclp | Microsoft Research Cliplets Project |
| xcode | Xcode Project |
| xcodeproj | Xcode Project |
| xda | Sencha Architect Project Archive |
| xer | Primavera P6 Project |
| xgll | EASE SpeakerLab Project |
| xise | Xilinx ISE Project |
| xjp | Jindent Project File |
| xmfg | MediaForge Runtime Player Distribution Project |
| xmk | Crossware Embedded Development Studio Project |
| xmp | Xilinx Microprocessor Project |
| xmpz | Miradi XML Project |
| xosm | PixInsight Project |
| xpj | Adobe RoboHelp XML Project |
| xpj | Micro Planner X-Pert Project |
| xpj | Akai MPC Software Project |
| xpr | Microsoft Expression Project |
| xprj | Monarch Pro Project |
| xproj | Microsoft Visual Studio ASP.NET Project |
| xrp | RationalPlan Project |
| xtc | XTrkCad Project |
| xtodvd | ConvertXtoDVD Project |
| xtv | Xtivity Project |
| xtwd | Twido Suite Project |
| xvct | VISE X Export Bundles |
| xwp | XMLWriter Project |
| xyp | QuickWave 3D Optimization Project |
| ygn | yGen Project |
| ymw | Yaskawa MPE720 v6 Project |
