Last time I went, I took a book I was reading, but discovered to my dismay that I could not in fact use the front light of my bike as a lantern due to the highly focused beam being totally rubbish for tent-wide illumination.
I looked all over for a decent solution, the best thing I could find being the Black Diamond Orbit lantern. It's a 1 watt LED powered by 4x AAA batteries. Here: http://www.blackdiamondequipment.com/gear/orbit.php
The problem is, it uses all those pesky AAA batteries, which I don't think are actually lightweight at all, and also, the thing itself is made of excessive material as it's more durable than it needs to be (for my application anyway). Plus, it costs money.
I came up with my own solution using crap I had lying around. Here is a photo of it unlit:

It's made from the following:
1 x 35mm film case, transparent and acts as a diffuser, cost = free
9 x 5mm white LED's, cost = nearly nothing, probably $1/£1
9 x 100 ohm resistors, cost = infantessimally small amount, probably 10c/10p
1 x Standard USB cable, cost = free
I drilled 3 holes in the case cap so that the USB cable could enter, then come out and loop back in, thereby giving me a hook to hang it by.
Here it is lit, plugged into my computer USB port:

So, why make a USB powered ultra lightweight DIY tent lantern you ask...
... Well I always have with me a Freeloader, which is a sort of universal solar charger and battery arrangement which can be used to power almost any portable electronic device. I have this with me anyway, to keep my phone alive, and charge rechargable batteries for my GPS. See the freeloader here: http://www.solartechnology.co.uk/produc ... loader.asp. I strap it to my bike while I ride around or leave it on the tent during the day (I have the additional 'supercharger' external solar panel option which is weatherproof and remote from the main electronics so it's safe if it rains).
The freeloader comes with a load of different plugs for various pieces of equipment which you swap onto the end of the little power cable that comes with it, one of which is a standard USB socket, into which I plug my lantern.

This should be enough light for me to read by, and I'm thinking of making a different version with just 3 higher powered white LED's, or maybe a mix of white/orange to give a warmer colour.
Just thought I'd share. Peace.