It's not necessarily all about power, i.e. 3 watt Crees. Here's a link to a blog by a guy who seems quite focused on the Orphek brand of LED & their fixtures. Many interesting reads here and although it's a personal blog he provides plenty of data in the form of spectographs in order to support his argument that the colour spectrum output of an LED can be just as, or more important than simply the power output.
Many options out there these days & more on the way. Personally I'm liking the MR16 and PAR38 bulb form factor. Pretty much plug & play as the PAR38 bulbs simply screw into any standard household fixture and the MR16 bulbs are designed as accent lighting running on straight 12 volts DC. This would make them perfect for a smaller nano or pico tank which you could run off a 12 VDC power source such as a deep cycle battery charged by solar panel. No soldering or special LED drivers required. In addition, if one of the bulbs goes, you're not looking at trying to repair an entire fixture or module. Simply pop out the old bulb & put in a new one, just like current lighting systems we use. I'd also venture to guess, if you used a bunch of them, this concept can easily be applied to larger tanks.
http://blog.captive-aquatics.com/cap.../led-lighting/