Online games:

Single Player:

• Full Screen Mario
Full HTML5 port of the original SMB.
• Little Alchemy
Make things, combine things and convert into a insane god.
• Little Alchemy 2
The same, but with autoplay music, updated graphics and effects

Multi-player:

• Chess
Play the classic strategic board game. You can challenge the computer, a friend, or join a match against another online player.
• skribbl.io
Free multiplayer drawing and guessing game, like Draw My Thing.
• Free Penguin
Club Penguin clone with everything free.
• Supermice
Transformice server that auto proclaims as the best one.

Online services:

File hosting:

• Doko.moe
Kawaii file hosting featuring anime girls.
• MegaUpload.nz
MegaUpload old site revival, it even fulfills its purpose.. It's not a joke!

Email providers:

• Cock.li
Email service with weird and funny domain names.

Web apps / tools:

Downloaders:

• YTMP3
Convert YouTube videos to mp3 easily.

Generators / makers:

• ezGIF
Online GIF Maker with multiple options.
• PHOTOMOSH
Glitch images, videos or webcam using WebGL effects.
• Da Button Factory
Easy and complete tool to make buttons.
• HIPSTER IPSUM
Lorem Ipsum style text generator.
• HTML Color Picker
Find HTML codes for colors.
• HTML Color Gradient Generator
Generate codes for HTML gradient colors.

Emulators:

• Browser screenshots
Check if your website is compatible with other browsers or devices.
• JSLinux
Run Linux or other operating systems in your browser!
• Deja Vu
This site let you emulate some 90's web browsers.
• oldweb.today
Another one to emulate old browsers, but this let you visit Archive.org websites.

Other:

• Scribens
Checks the grammar of any English texts and finds spelling mistakes.
• Nu Html Checker
You can check here if your website source codes has mistakes.

Web resources:

Graphics:

• GifCities
The most complete Geocities gifs searcher.
• FreeBackgrounds.com
Tons of free backgrounds for you site.
• Bonnibel's Graphic Collection
• A bunch of cute graphics and gifs for your site!
• The 88x31 Collection
The largest buttons database on the net.

Music:

• midiworld.com
Free MIDI music to download.

Scripts:

• RV's free Javascript/DHTML effects
Free cool HTML effects for your website.

Other:

• DaFont
Large collection of fonts, searchable by licence.

Miscellaneous:

• Cameron's World
A web-collage of text and images excavated from the buried neighbourhoods of GeoCities.
• callhim
Put your name and see what happen next...
• End of the Internet
The very last page of the Internet.
• WINDOWS 93
The best version of Windows that ever existed, with lots of things to waste time in the coolest way.