Unfortunately, it's likely that some of these links will die, so I'll try to give multiple sources.
Manga
Anime
Nyaa Magnet: All 195 episodes.
Nyaa Magnet: All 12 OVAs.
Youtube: Alternatively they've been uploaded to youtube (with the OVAs) (check this playlist for the missing episodes).
Project-GXS DDL: You can also DDL it all from Project-GXS website.
Nyaa Magnet: All 12 OVAs.
Youtube: Alternatively they've been uploaded to youtube (with the OVAs) (check this playlist for the missing episodes).
Project-GXS DDL: You can also DDL it all from Project-GXS website.
Music
Nyaa Magnet: Complete music box 15CD in flac.
Open directory: DDL of the Complete music box + 2 other albums.
Youtube: UrusUrusei channel, A lot of Urusei Yatsura music and misc releases.
Youtube: Dr. Butte Fatman playlist, almost all UY music releases.
Open directory: DDL of the Complete music box + 2 other albums.
Youtube: UrusUrusei channel, A lot of Urusei Yatsura music and misc releases.
Youtube: Dr. Butte Fatman playlist, almost all UY music releases.
Misc.
E-hentai (sfw): UY '85 calendar.
E-hentai (sfw): UY '86 calendar.
E-hentai (sfw): UY '91 & '92 calendar.
Websites
Ironmouse: "CG shrine", fan-arts of Lum.
Sailorlum: Fan-site, I don't remember how to go pass the front page.
Furinkan Tomobiki-cho: The Urusei Yatsura website, active since '96
Tomobikicho-4cho-me: Website in Japanese only, but it's worth digging through.
Animezu: A few cel of Urusei Yatsura.
