Links

Current Status of Aliquot Sequences < 3,000,000 — Current status of Aliquot sequences with starting value < 2 million. Contains reservations, current drivers/guides, and factors of the last known term of each sequence.

mersenne.ca — Sister site to www.mersenne.org and www.mersenneforum.org, contains all known factors of Mersenne numbers (up to M9999999967), as well as status of current efforts to factor them.

Midtown Madness 2 Xtreme — Contains various addons (cars, tracks/cities, mods, etc.) for the Microsoft games Midtown Madness 1 and 2, as well as a discussion forum. They don't take requests, however; you'll have to find another site to request 3D models from, if they don't already exist.

Syd's Number Factorization and Primes Database — Contains billions of prime, probable prime, composite (incompletely factored or no known factors), and unknown numbers.

Million Digit Base-N Calculator — Useful calculator that can also be used as a base converter. Currently has a bug where the output isn't displayed at all.

DeviantArt — Art website; has some useful resources. Some stuff requires you to login/register to download; some stuff requires payment (points or real currency); some stuff is completely free. Either way, make sure the license for any given resource is compatible with what you're making (renders, game mods, etc.), so you don't get a warning from the artist who made it. Also, it's a good idea to include a credits.txt in the zip/rar/7z archive you distribute the mod in, as some licenses require it.