Here are a few that haven't been mentioned (or at least, that I haven't seen here yet):

1. Add audio to EVERYTHING and sell it as a new package. If you have a video, pull the audio. If you have text, record the audio, etc. It's a quick, easy way to add more value.

2. If you have a blog, you could put together some of your most popular posts (with audio), and toss in a bit of new stuff for a quick, small product. The same can be true with popular e-mail blasts, depending on your style of writing.

3. Buy a bunch of stock art with a commercial license (make sure that the license allows for this!) and bundle it all together and sell the bundle for a WHOLE lot more.

4. Make a video series based on previous blog posts/ e-mail blasts, and sell that as a product.

etc.

I have yet to do any of this myself, but basically, it's just a lot of re-purposing. XD lol

I hope that helps!