Various Chassidim have different customs regarding special occasions where they don't say Tachanun.
[I know this because sometimes I daven in a Vishnitz shule and, upon asking why they didn't say Tachanun one Chassid answered that it was a special day for Vishnitz (I forgot what exactly... maybe a Yartzeit of a Rebbe)]
For instance Chabad chassidim don't say Tachanun on a day where one of the Chabad Rebbeim was released from prison. (= Yom HaGeulah) [see Sefer HaMinhagim p16, Hayom Yom 19th Kislev, 12th Tamuz and 13th Tamuz (search for the word 'Tachanun')]
What special days/occasions do other branches of Chasiddim have on which they don't say Tachanun?