Plus Gas?
WD40?
GT85?
Heat gun and a sharp tap?
Stuck seatpost?
50/50 acetone and atf fluid, or commercial grade plus gas but sometimes heat and shock (hammer time) tactics work best
KY Jelly
Scottish Snow, manages to get you cold and wet just looking at it
Depends what yer penetrating
Cant see the missus being amuzed by a 50/50 acetone atf mix
A bottle of Pinot Noir or a pint of gin
[img] 
[/img]
Definitely Plusgas...probably followed by a whack with a hammer!
Coke!
Bottle of buckfast? Never tried it but the name suggests it is good
PlusGas. If it can free suspension bolts on a 23 year old Mazda, it can free anything.
plus gas is good , but im a cheap ass bastard so used ATF and acetone as somafunk says ....
got my 30 year old landy apart with that and somme precise use of the impact wrench.
double post
I've always found wd40 to be perfectly good at a reasonable price.
If you can wire brush first then apply some heat so the liquid is drawn in further when applied I've found it works better, also tightening bolts a tiny smidge before unbolting and only going a tiny bit backwards at a time then forwards and back to clear the threads also helps prevent breakage or rounding off.
WD40 now make a range of "specialist" lubes in addition to the traditional one. Their penetrating one is good, better than the professional recommended stuff I ordered off the internet previously, and it's available in Halfords.
Diesel.
Better than all the others by far.
Practical Performance Car did a group test a while back, they found gt85 was very nearly as good as plusgas. (though nothing was as good as soaking in diesel). Those guys know everything there is to know about manky bolts stuck in shitty scrapyard-worthy cars so I trust that judgement.
Depends on what's stuck? Penetrating oil won't do much for aluminium oxide - ACF50 would be better for a seized seatpost.
It's for general use but specifically at this moment, for the potential removal of steel bolts from a woodburner.
brake fluid
double post
Practical Performance Car did a group test a while back....
By far my favourite car magazine.
Sorry, nothing to add to the penetration debate.
RP90, if you can find it. It's bloody amazing stuff.