Mine isn't an antique yet, but I'll throw in the indestructible, green, steel, Stanley Thermos bottle.
It doesn't have to be an antique, just a piece of kit whose design can't really be improved upon such as the classic fisher space pen. (I sometimes still use an 20 year old but heavyweight Stainless Steel Coleman's Thermos Bottle)
Technology is advancing so rapidly I'd guess my flashlights and headlamps may not have batteries available for them in 50 years.
I can't really see the AA or CR123 cell format disappearing anytime soon simply because of the huge number of devices that rely on the format. NiMh cells are rechargeable for use over 1000 times and the latest low discharge technology such as the Sanyo Eneloop may be able to hold some charge after even 5 years. So I guess there might even be some low discharge technology NiMh AA cells still around in 50 years time (I will have to research this further though). Freshly manufactured primary Lithium cells stored in cool conditions should still have more than 50% of the initial charge after 25 years.
Many of the items already mentioned have stayed away from the electronics side but perhaps with the introduction of completely solid state SSD hard drives many computers such as this low power PC (just perfect for solar powered PC) may still be working 20-30 years from now.
http://fit-pc2.com/wiki/index.php?title=Main_PageEven with clothing, Ventile cotton material is still usable after 25 years especially if looked after and can be repaired more easily than Goretex (even Goretex I have found to be remarkably durable as I still own and use a 12 year old Berghaus Extreme 7000 Jacket during winter walks)
In the UK we are about to have a decade of austerity (mass unemployment not seen since the 1930s, with the politicians rhetoric already suggesting that the mass unemployed are just lazy layabouts even before they are made redundant in the next couple of years through government spending cuts
) and I'm looking to replace items which have in built obsolescent designed into them or are difficult to repair without having to rely on the manufacturer who might not be around in 10-20 years time.
As an example, one of the most annoying and frustrating issues I have at the moment is with companies such as bicycle parts manufacturers Shimano and Campagnolo for replacements for 15 year old bicycles
I don't need 10 speed freehub gears and 10 speed chains I only need 7 speed gears and shifters of the same quality that was made in the late 1980s and early 90s. I need conventional headsets (not Ahead) and I don't need carbon forks because I prefer steel ones with threaded columns with old fashioned quill stems. (these companies have designed their components for designed obsolescence after a couple of years, i.e throw away parts that wear out quickly). My bicycle frames, which I made 15 years ago still have a 50 years of life still in them.