The Mugen power battery. Ridiculously expensive and ugly, but for me it was worth it. Now I can finally hard use* it over the day and there's still 30% power left before going to bed.

*hard use means: 2-3 hours surfing on 3G (3G-only is on over the day) while listening to webradio at the same time, playing up to 1 hour with the device in the evening... Device is overclocked (currently 950Mhz), eMail is being fetched every 5 minutes, display is on highest brightness and checking for internet connections is set to each 10 minutes. WLAN device is on all day with medium powersaving (low doesn't work to good here). Plus all the things I can't remember right now
I know, I could tune it to use less battery, but right now I'm to lazy, just because I can The highest battery uptime was 3 days with better settings.