Links:

This isn't just the only source of C64 stuff available. There are many other places which contain information about other aspects, such as either specific game information, or other aspects.

Informational Sites (Docs, Help)
Project 64A site which was setup to store electronic copies of C64 Hardware & Software Manuals. It's a very useful site, and you can also request manuals to be added there.
comp.emulators.cbm NewsgroupThis newsgroup is one of the best places to ask for help if you're just getting started with using C64 emulators, and there's a comprehensive list of sites which is frequently posted, which might help with finding rare games.
comp.sys.cbm NewsgroupThis newsgroup deals with similar topic as with comp.emulators.cbm, but emulation discussion is not something that should be posted about.
comp.binaries.cbm NewsgroupOnly used for releasing freeware/public domain software, you can find some useful programs here!
The C64 Web-RingThe centre page for the Commodore Web-Ring, which contains heaps of other Commodore 8-Bit related sites.
The C64 Game GuideThis site has a massive database of info about almost any C64 game release, you can downloads the tunes to run on SIDPLAY (a package which lets you run C64 music of UNIX, Windows 95/NT, and Mac based systems), and has a section where you can ask questions about any game. Very well done!
Fairlight Tools PageThis page is a huge list of C64 related tools forthe PC, Amiga, UNIX boxes etc.... Useful if you're in need of a utility, is also has some 6502/6510 cross assemblers so you can write C64 ML programs on your machine and then port them across to run them.
The Commodore Knowledge BaseSet up as a resource which holds general information about all Commodore Machines, the CKB is useful if you're trying to locate specific information on programming, or even game playing (there's a small list on the site)
The FridgeSimilar to the CBM Knowledge base, but the focus is on programming routines, which are stored, all for general public usage in their programs, in addition to some useful programs, such as a 3D graphics routine, and a music program.

Search Engines(Any files):
C64 SID searchThis site is a search engine of SIDtunes, (i.e. C64 music) to run on SIDplay. Although not as simple to use as other search engines, this one is very powerful, and useful if you're looking for a tune to listen to!
Commodore FTP searchA nifty search ngine, used fo locate files of any type (demos, documents, games etc..), which searchs over a vast range of C64 FTP sites. Useful, if you're looking for a specific title
C64 SearchIf you're stuck looking for a certain game, try this! Indexing around 10 of the major C64 game archives, a search with this should yield you a quick result!

Game Download Sites (Emulator Images):
arnold.c64.orgFormerly arnold.hiof.no, this site is one of the biggest C64 FTP sites around. It contains all of the Blast volumes (340!) and a huge collection of C64 games, all ready to run on emulators (sadly, not all of them were copied properly, and there is a percentage of the which are faulty), as well there are emulators, plus utilities, and a library of SID files for SIDplay (a program which allows the playing of C64 music on a high-end machine).
Commodore Zone A large site containing MSDOS/WIN95 Emulators and games, and plenty of links to sites where you can get Amiga games and emulators.
Martin Pugh's C64 pageOne useful page if you hate FTP sites! He's written an index of the entire catalogues of the arnold.c64.org and The Digital Dungeon sites, and linking the games directly to them.
Blast From the Past HomepageThe offical page of the brilliant Blast from the Past Series. Currenly, there are 340 volumes available, which consist of a large variety of C64 games.
C64 Internet ArchiveThis packed site offers many games (both in standard and TAP formats, in fact, all the TAP's available online!), and all the Sids from the High Voltage Sid Collection (sorted for individual downloading). The amount of info here is huge, and covers all areas of emulating a C64 (including copying from a real one to a PC)
CCS64 Tape filesThis site carries a small collection of files in the TAP format (which allows proper loading, and software protections to be copied over as well), there is also some help in converting tapes into this format on the site as well.
Jez's PageJez's page carries some rarer titles which are available for the C64!
The SuperCPU homeCarrying a small selection of games and demos which will run on this C64/128 accelerator (boosting speed up to 20Mhz!), you can grab some old titles, which are able to run correctly with these upgrades!
Commodore WireA good news site, concerning with details other than just emulators, this site, offers cheats, and a good selection of game demos to try out
Commodore ApocalypseNicely presented site, with a fair amount of games, plus a top 100 list, and some C64 related puzzles, which are quite tricky!

Game Download Sites (Non Emulator Images):
Sharkbite Online!Another site with lots of games, however, you'll need a copy of Lynx , the LHA decompresser and the Star Commander to use them, because the site wasn't designed for emulator users.
The Digital DungeonThis site has a large selection of games, but, like Sharkbite Online, they're in C64-formats, designed for a C64 who accesses the net through a UNIX connection.

Game Download Sites (Author Freeware):
Ian Bell's Elite PageMaintained by the co-author of this classic, here you can grab any version of it (ready to run on emulators), and grab a copy fo the full manual (HTMLalised!), so you can enjoy this classic (again!)
Beam Software's 8-bit Software PageAlthough their titles weren't too well known (execpt 'Way of the Exploding Fist'), Beam have gathered their collections of C64, Spectrum & Amstrad CPC games, and put them up here for free download.
Cosine SystemsA fairly newish group, you can find their games and demos online here

Sid Tune Download Sites:
C64 AudioUnlike other sites, this contains neat remixes of classic C64 Sidtunes in MP3 format.
The High Voltage SID collectionThis entire collection contains almost every single Sid tune which can be located online.

Fan Sites:
The Last Ninja seriesThis site gives info on the classic series by System 3, and allows you to download them for emulators (or the PC), and even offers snippets on the forthcoming Last Ninja 4!
Creatures IIThis site offers a brief overview of one of the best C64 games produced in the later days.
UridiumDedicated to the classic Andrew Braybrook shoot em up, this site covers Uridium on all platforms, although there is more detail on the C64 version
Iain's C64 pageOffering a decent range of articles from the major C64 computer magazines, and although having a huge number of games, the section about the computer magazines is what earns it top spot!

Anti-IDSA Sites:
The Campaign to Leave Emulation Alone [Plus an R]The major Anti-IDSA campaign online, this site lists permitted titles for distribution and members who do/don't support the IDSA action, and up-to date news over the crisis.
The IDSA Boycott PageSet up to convince emulation fans to stop buying titles until the crisis is over, this site is also filled with some news regarding this aspect of the operation.