Solar Sponge
Solar Sponge is a soak mutation which gives resistance against burn, radiation, and laser damage. It also lets you heal when you're hot (healing is determined on temperature), and gives a maximum bonus of +2 to Brawn/Brains while in the sun.
Location
Solar Sponge can be found at ground zero, in the Wasteland
Effects
This mutation grants:
+1-3 burning resistance
+2-2 irradiation resistance
+2-3 laser resistance
Healing effect when hot
+2 to Brawn/Brains when in the sun
Drastically reduced chance to acquire a Mental illness when using Carrie
Advantages
Apart from the bonus burn/irrad/laser soaks, you'll get a fairly good Brains and Brawn bonus when in sunlight.
You'll also get healed at high body temperatures, which can go up to +6-7 per tick in hot places like the ant colony or summertime Coventry.
Other neat bonuses are taking no damage from being on fire and having a drastically lowered chance to contract Carrie mentals.
Disadvantages
While you're immune to damage from being set on fire, you will still suffer the debuffs to your weapon skills.
The +2 Brw/Brn bonus while in sunlight is unreliable compared to Yeti Fur's permanent +1 Brw.
Despite being healed by heat, it is still possible to get TOO hot if you're on fire, debuffing you like normal and possibly even hurting you through your soaks.
Note that this is the heat debuff, which is separate from and in addition to the sizeable on fire debuff, so don't go overboard.
Appearance
His/Hers/It's glowing skin has a bleached-white look to it.