Ok, this is Storage Review, so why not make a storage guide/review - for free online storage.
Please add information and experiences you had and I'll amend this post accordingly.
Many companies offer free online storage - advertisement supported and completely advertisement free.
With Photography I had to look into free storage twice for fellow maths students.
Please note that this should not encourage sharing of copyrighted material where you do not own the copyright.
Here are a couple of Free Online Storage offers:
Advertisement supported - apparently to some people the download link isn't obvious - it also gives you a delay before the download is allowed.
One has the choice between registering for an account that allows file management (e.g. deletion) or "just uploading".
File sizes limited to 200MB
A free Advertisement supported service - again, a delay before downloading begins.
If I remember correctly during testing I found that some of the advertisements may be - hmm, heading towards inappropriateness.
File sizes limited to 300MB if I remember correctly
Registration requires quite a lot of information but for this you get direct hotlinking to the files - or a page that displays advertisements - files are subject to a download bandwidth limit of 5GB per 30days - then this is reset.
File sizes limited to 1GB
http://skydrive.live.com/ - Recommended
The Windows Live Skydrive - to upload files a Windows Live or hotmail account is required.
Live accounts get 25GB of storage - I used this to deliver photographs to students on my maths course - apparently downloading can be a bit dodgy at times.
Filesizes limited to 50MB
One of the most commonly known filesharing/file storage sites - the one time I tried to use it I didn't manage to upload any files...
Synchronizing and Backup:
Allows to share, backup and synchronize files - not sure how it works - the site isn't all that clear to me.
Edit: This will be replaced by Live Synch - currently available as a Beta in the new Live Essentials. http://explore.live.com/windows-live-essentials-beta
http://www.box.com - Recommended
Box.com is similar to the older established Dropbox - only backup and sharing of files. Paid for plans also allow for collaboration (not sure about free accounts.)
Free accounts have 5GB of space (offers via BlackBerry included 10GB and Dell 50GB) - more is available on paid for plans. Files are limited to at most 250MB of a free account.
http://www.dropbox.com/ (Wide user base - established service)
A client based system that let's you backup and synchronize files and also allows sharing, however accounts that use excessive bandwidth sharing files, may have the sharing of that file suspended.
Free accounts offer 2GB of space.
http://www.copy.com - untested
Mentioned in this thread in a new post - cannot comment on quality. Offers free and paid for accounts.
Google documents which offers 1GB of storage for any filetype.
250MB maximum file size.
http://www.flickr.com - Recommended
A popular photo sharing service from Yahoo - a Yahoo Account is required.
Free accounts can display 200 images - if you upload more the oldest are hidden - they are not deleted.
Free accounts are limited to a maximum displayed image size of 1024 Pixels on the longest side, and 3MB per image (think so), as well as a monthly upload of 100MB.
Google's image sharing site/system requiring Picasa software and a google account it allows users to share images.
I do not have information about any limitations handy at the moment
http://www.imageshack.us/ - Recommended
A free image sharing site - image sizes are limited, and the site offers the choice between accountless image sharing and signing up for an account.
Speeds can be a bit slow at times.
A similar service to Imageshack - photo sharing for free, limited image sizes, with or without an account.
Please add any other services you know off as well as some personal experience and information.
Small files that need constant sharing and permanent availability can be shared effectively via Wordpress - especially if they have a story to go with them.