I have used Blu-Tack a lot, and even though its reusable, it does wear out, and I got tired of paying around $10 for a pack, so started to look for other putty's that would take its place.
I will go to the store today cause I need more anyway(got a lot of models with drying glue on them right now!!

) but I found this small package of Thumbtack putty in my PX(the military version of a department store/Walmart, for those that don't know) for $1.29!! It is pretty much Blu-Tack with about 3/4 of the usage life, comes in a 2"x6" sheet with squares already dimpled in for easy tear apart, and is normally used to hang pictures or wall decorations without nails(very important in military barracks and family housing!). For the price, its unbeatable!!! It holds just as well as Blu-Tack, and it peels off really easy so there is almost no mess!
An added feature over Blu-Tack, simply because I don't think anyone would use there expensive Blutack to do this, is that it works just like modeling putty and simply needs a bit of superglue applied to the outside of it to turn it as hard as a rock!! So its also a cheap, albeit less permanent, replacement for Green Stuff. Be warned though, it only works well in this role if you are using small amounts, as the superglue will not penetrate deeper than 1/4" on it so it won't harden the whole glob if its a big one, but its great for a hole or gap filler, and minor conversion work!
There are many brands of it out there, and its cheap, so I hope this helps a few people save a few bucks!!!
Edited for grammar and spelling errors.