Doom 2 - ZPack - "Vestibule by Captain Toenail"
First level of ZPack "Vestibule" which was mapped by Captain Toenail
ZPack was a collection of levels made for ZDoom. I think it's up on Tormentor667's website. I'd say definatly check it out, it's hard, but very fun and very well put together. (http://www.realm667.com/)
Oh yeah, i experienced a glith on ZPack's second episode, after completing E2M2 - i went straight to MAP02 of Doom 2, it was said that the glitch got fixed with a new PK3, although i never did find out.