Upload large files in .NET

Solution 1:

1) Is it possible to resume a file upload on the client without using flash/java/Silverlight?

No. The actual HTTP protocol itself does not support resume of partial uploads, so even if you did use flash or silverlight, you'd still need to use something else like FTP on the server.
I've "solved" this problem in the past by writing a custom client application in C# which broke the file down into small chunks (2meg), transmitted those separately, and then the server combines them all back together.

2) Does anyone have some code or a link to an article that explains how to write a .NET HTTPHandler that will allow streaming upload and an ajax progress bar?

While this doesn't solve the 'resume' problem, I've used SWFUpload on the client side and it worked brilliantly. It provides a smart file browser (where you can prompt the user for only jpeg files, etc) and upload progress tracking, all without needing to modify your server at all.

Solution 2:

It's not possible to resume an upload using standard HTML file input control, because the whole file gets submitted with the HTTP request.

I've used NeatUpload in the past, which gives you a progress bar. It's under an LGPL license, so you don't need to pay for it and it's open source.