| xrmcache | XrmToolkit XRM Cache |
| xrmsuo | XrmToolkit CRM Credentials Data |
| xsc | Xscript Script |
| xsc | Dataset Designer Internal Data |
| xsd | Altova XMLSpy XMD Schema Definition Data |
| xso | Expresso Project |
| xsql | Oracle XSQL Servlet Server Side File |
| xss | Dataset Designer Surface Data |
| xst | IBM WebSphere Query Template |
| xsu | Fortran 77 Library |
| xsw | Milnta APL Transfer Function Data |
| xsy | Unity Pro Exported Variables |
| xsym | CodeWarrior xSYM File |
| xt | Xdebug Trace Output |
| xtml | XML4web HTML Document |
| xul | XML User Interface Language File |
| y | Yacc Source |
| yab | Yabasic Source Code |
| yac | Yacc Source |
| yacc | Yacc Source |
| yajl | YAJL Script |
| yang | YumaPro YANG Module |
| yaws | Yaws Server Dynamic Page |
| ygl | YAGL Script |
| yin | YumaPro YIN Module |
| yml2 | YML 2 Script |
| ymx | Yukes 3D Object |
| yo | Y86 Assembler Object |
| yobj | Yukes 3D Object |
| ypp | GNU Bison Source Code |