LupintheThird.com - One of the original Lupin fansites, created by the old guard Lupin expert himself, Mr Reed Nelson! It's sadly a bit barren now, but in its earlier forms it was a really great site.
The Lupin III Encyclopedia - A very good resource for all things Lupin. It hasn't been updated in quite a while, but it covers just about practically every anime and manga production pre-2012, as well as anime adaptations of other Monkey Punch-related work like Alice and Mankatsu.
The LupintheThird.com Forums - A great place to chat with fans across the globe!
Lupin Central - A great news resource, and along with its Tumblr page (see below), another good place to connect with other fans.
Lupin Central Tumblr Page - Basically the above, only on Tumblr (it came first)! A great place in general.
The Lupin the Third European Page - Another good fan resource, this one is rather outdated too, but it has lots of cool downloads and cover scans, and is bilingual! In English and Italian.
Lupin III: Presented Without Commentary - Another Tumblr page, focusing on out-of-context screenshots from the anime. If nothing you find here makes you laugh, something is seriously wrong with you.
Lupin III Confessions - Another Tumblr page, here you can confess your Lupin secrets!
The Castle of Cagliostro - And another Tumblr page, this one revolving around Discotek's upcoming re-release of the great Castle of Cagliostro!
The Lupin III Network - The official Japanese Lupin site, which is only in Japanese.
The Studio Ghibli Museum's Official Lupin Site - Another Japanese-only site.
TMS Entertainment's Official Lupin Site - Another Japanese-only.
The Official 40th Anniversary Site - A site made in celebration of the 40th Anniversary of the anime. Japanese-only.VAP Video's Offical Lupin Site - A website containing information on Lupin home video releases. Japanese-only.
Amazon.com - A great place to get Lupin goods from!