At the moment, I'm pretty attached to my Brookstone 92-Hour Rechargeable Battery Pack. It cost me $80 and hides a 3000mAh battery in a very small and light weight footprint... and can almost recharge my iPhone 2x at best. It's a bit tricky to find in the UK, so you'll have to do some searches on Amazon and eBay.

Before that, I was using the Mophie Juice Pack Powerstation. It holds more of a charge at 3600mAh and does recharge my iPhone 4 twice over. It's bulkier than the Brookstone but has some advantages (namely the cable).

Today, I received my New Trent Dual-Port Rechargeable Battery Pack and I must say, it looks really good. The price was excellent - £25 here in UK - and it has a 5000mAh battery with two ports - one for 500mA recharging, the second for 1A recharging (e.g. iPad). I haven't used it yet, so I'm going to post more after I've taken it on the road... but I'm really excited that not only was it well-priced, well-sized and packs a lot of power compared to the others... but it also recharges by micro-USB cable instead of something a bit more proprietary, like the other two. In particular, the Brookstone comes with a cable that I'd hate to lose, which would leave me dead in the water.