If you're looking for an inexpensive softbox option, homemade lighting modifiers like Studiolighting.net's DIY foam core softbox are a good way to go. But if you need something with a cleaner, more professional look, I like Lumiquest softboxes and snoots. They're foldable, under $100, and washable. To mount a portable flash to a light stand or tripod and soften the light at the same time, I recommend the Lastolite EZyBox Hotshoe ($150 street). It's a folding softbox with a flash mount built in to take your standard on-camera strobe. The mount even swivels to offer a more direct line of sight between a flash's built-in IR receiver and a camera-mounted transmitter. I've found that the Lastolite is equally useful for close-up work and location portraits.
_ —Matthew Panzarino