He was in the past credited with inventing the Sequence, by which I mean the prolongation of the Alleluia at Mass which, they think, grew out of the jubilus, or very long melisma that prolonged the singing of Alleluia on feast days, simply by adding a poem to be sung to the same melody.
The Mediæval Mass had a Sequence almost every day, but most of them were excised at the Renaissance (though it was the Renaissance that beatified Notker in 1512), and I don't think any of the surviving ones are by our friend. Rather unaccountably Bugnini, in his iconoclastic way, changed the position of the Sequence to before the Alleluia in the 1970 missal, thus severing its connection with the ancient past, and, indeed, its very raison d'etre. I really miss the final Alleluia after Amen in the sequence, especially after the Easter and Pentecost sequences—I find the concluding Alleluia melodically and emotionally very satisfying. It feels bald without it.
Notker is also famous for writing one of the two biographies of Charlemagne (the other being by Einhard).
Here is Notker's Sequence for Pentecost:
Adsit nobis gratia
Quae corda nostra sibi
Expulsis inde cunctis
Spiritus alme, illustrator hominum,
Horridas nostrae mentis
Amator sanctae sensatorum
Infunde unctionem tuam,
Clemens nostris sensibus.
Tu purificator omnium
Purifica nostri oculum
Ut videri supremus
Genitor possit a nobis,
Mundi cordis quem soli
Cernere possunt oculi.
Prophetas tu inspirasti, ut praeconia
Christi praecinuissent inclyta.
Apostolos confortasti, ut trophaeum
Christi per totum mundum veherent
Quando machinam per Verbum suum
Fecit Deus coeli, terrae, maris,
Tu super aquas foturus eas, numen
Tuum expandisti, Spiritus.
Tu animabus vivificandis
Tu, aspirando da spiritales
Tu divisum per linguas mundum
Et ritus adunasti, Spiritus.
Idololatras ad cultum
Ergo nos supplicantes tibi
Exaudi propitius, sancte Spiritus,
Sine quo preces omnes cassae
Creduntur et indignae
Tu, qui omnium saeculorum sanctos,
Tui numinis docuisti instinctu,
Ipse, hodie apostolos Christi
Donans munere insolito
Et cunctis inaudito seclis,
Hunc diem gloriosum fecisti.