France:
Les 2 Alpes: from middle of june till august, huuuuge park but very crowded and very bad snow
Tignes: july and august, not a good park but a steep long run to ski on
no more summer skiing in La Plagne, Val Thorens, Alpe d'Huez etc...
Switzerland:
Les Diablerets / glacier3000: until middle of May, sick park, uncrowded
Saas-Fee: from middle of july until winter: sick park, uncrowded
Zermatt: year round but park crew is only there july-august-september-october, the highest (3899m), best snow, uncrowded, sick park
Engelberg: until end of May, no park
there is no more summer riding in Verbier, Laax, St. Moritz, etc..
Austria:
Kaunertal: till end of may, sick park, spring classics
Dachstein: from the end of may till the middle of july, sick park
Kitzsteinhorn / Kaprun: year round, sick park
Hintertux: year round, sick park
there are a bunch more glaciers open in Austria (Pitztal, Stubai, ..), but not with a park I think.
Italy:
Val Senales: park in june and july
Cervinia: accesses to Zermatt
Passo Tonale: ?
Marmolada glacier: ?