Mindofme - those ebay boxes like the above on are quite good; diffusion is nice and even and they don't suck up too much light so you're not having to work on full power all the time. I think some come with a double diffuser so you can alter the amount the softbox diffuses by. The ones with a ball-joint offer angle options but in my case, the ball-joint only lasted a few weeks so i ended up getting a fixed 90˚ speedring off FITP and using an angle attachment on the top of my lightstand.
If you are feeling flush and want a big softbox then have a butchers at the Westcott Apollo 28". I bought mine from premierink as a kit
(stand, speedlight attachment, softbox) for £135, which was the deal at Focus2011 that's been carried on.
The quality of light from the Apollo is amazing and it's a much more handy solution for me (carrying kit around) because it's based on an umbrella design so packs down very slim and compact.
With the Apollo I do find that when working outdoors and I'm pushing my speedlights a little more because of the larger size and light spread when compared to the 12"x12" boxes but for full and half-body shots it's great. Plus you can get that 'window' effect thanks to the bigger size, creating much softer light to recreate daylight.
If you do go for a standard softbox with the slot-in speedring, a good tip is to glue velcro on the inside and outside so when the speedring is clipped in, the Velcro wraps around to give you a bit more security. If you're using outdoors you'll soon find that a gust of wind can pop the softbox off the bracket quite easily.
