• You Send It - yousendit.com - this is one of the first web-based file transfer services I used. It's fairly reliable and widely used. The file size limit is 100MB. You select a file to upload to their servers, specify a recipient by email address, and they receive a link to download the file off of the yousentit webservers. These download links usually stay active (accessible) for around a week.
• Send Space - sendspace.com - much like yousendit except maximum file size is 300MB.
• Streamfile - streamfile.com - like the two above except the recipient can start downloading while you're still uploading. I am not sure if this is a significant feature, but it is pretty neat. Maximum file size is 150MB. Download links expire in 24 hours, which is a disadvantage. Still, if your recipient is anxious to receive a large file, this can start the transfer the fastest.
• Dropbox - getdropbox.com - Dropbox is a behind-the-scenes desktop application that works in the background on your Windows or Mac. Basically you have a shared folder that gets synchronized between all the computers that you run it on, and you also get a web-enabled folder inside that you can make web links to from your desktop if you desire. This service is best for synchronizing files across many computers, so it might not be optimal for one-off use, but there are many significant advantages to this software which makes it worth mentioning, such as seamless desktop integration, shared folders between accounts and 2GB of space for free. You can upgrade to 50GB or more for a paid membership, though I have found I haven't needed to.

If you use another service I'd love to hear about it in the comments area - or connect with me @davingreenwell. This is an area ripe for competition so new services tend to pop up and word of mouth is how I usually find out.

This post was originally my part of a discussion on the GDC Listserv. If you're a designer and not yet on it, I highly recommend getting involved with the GDC's active and resourceful community.