| TOM | object-oriented programming | 1999 | — |
| TurboWarp | — | — | — |
| UCBLogo | — | — | George Mills, Michael Katz |
| Unicon | object-oriented programming, procedural programming | — | — |
| Unlambda | esoteric programming, functional programming | 1999 | David Madore |
| XBLite | procedural programming | — | — |
| XL | multi-paradigm programming, imperative programming, functional programming | 2000 | Christophe de Dinechin |