I'm pretty much overwhelmed by the huge selection of laptops and tablets out there. I'm looking for something under $500 that'll charge off of my Shimano hub. If anybody here has one they think is just the cat's meow I really like to hear about it.

A dynamo hub produces about 3 watts, maybe more. That's not enough to charge even an iPad, much less a laptop. An iPad needs about 10 watts. And that's not even getting into conditioning the current and converting it to the proper voltage, which will reduce the amount of available power.

Agree with noglider,
To my knowledge the vast majority of laptops use a 12 to 14 volt battery. The vast majority of tablets use a 4 volt battery. Your best chance of success (slim) will be with the smaller 7-8 inch tablets or the larger cellphones.

You may want to ask your question in the "Touring" subforum where you may get feedback from riders that have done, or are doing/planning weeks/months long rides, many in remote or third world countries where reliable power is problematic at best, if they are aware of any dynamo hubs capable of supplying enough useful output for a tablet or laptop.

I don't remember about laptop batteries, but the chargers are generally 19 or 19.5 volts.

How often will you be away from an outlet? It is conceivable to power a laptop with human power, but not with a dynamo hub.

Only thing I would add is that a laptop has the advantage of swappable batteries, allowing you to in theory carry a spare battery and use that until you can get the other charged. That said, these days, a high-end smartphone can do much of what you probably want if you're touring and you'll get a lot more life out of a single charge. Another advantage is their charge cycles are going to be shorter than bringing back a laptop battery from zero.

OK guys, you've really rained on my parade here!! I'll have to consider different options.

Rained on your parade or brought you back to earth? If you want to fantasize about a human powered computer, go ahead. You'll do well to consider the laws of physics.

Get yourself a nice big battery pack such as this and a 7" tablet (my personal recommendation is the Google Nexus 7). Screen power is the biggest battery draw. You can charge the Nexus 7 almost 3 full times with that battery (there are bigger ones around). Depending upon how far you are going between charging sessions and how much use of the tablet you have, you can determine the size/number of battery packs needed. I have an older version of that Newtrent battery and it is a champ. 2A output means you get the full 10W charging. Good luck.

If you already have a bike headlight with a 7.4v battery you can use this USB adapter to turn your battery into a powerbank. You could run a large cellphone or small tablet (iPad mini) all day and recharge on your overnights.

I haven't tried one, but how about a solar charger for a smartphone or a tablet?

I have a Nexus 7 tablet but I don't take it out of the house much. Seems too fragile. I got cheap smartphone that I didn't activate on a cell service that I use as a mini wifi tablet that is easier to take in a backpack or bike pannier. It's a ZTE Flash that I got for $99 (kroger iwireless program). It's got a nice IPS display screen and a good 12mp camera. I got a hard shell holster case for it that protects the screen when I toss it in my backpack. I tend to use the Flash more than my tablet lately.

A used smartphone with a bad ESN would be good for this use too.

Lucky to charge a Smart Phone over a few days riding ..

Solar? panels of a couple square feet at least..

kept in bright sun and at an angle near perpendicular to the sun ..

If you are deadset on charging a tablet/laptop from a dynamo, get hold of these two items:

V72 Universal Laptop Battery (but I'd consider a solar panel rather than a dynamo)

and this too:

Charge a V60 from USB? It is possible, but? | Voltaic Systems ? Solar DIY and Device Charging

I am in no way affiliated with Voltaic.

I believe a iPad will charge albeit so slowly that a screen warning may pop up stating that its not charging.
Saying that, Apple Macbook Airs, iPad and iPad Mini boast a 10 hour battery life which is pretty much up there as far as what your looking goes.
Add in a high capacity battery and you extend this.
I've got a Macbook air I've just gotten to accept a battery via a cable from Ebay.
These:
ebay.com.au/itm/T-Head-Power-Bank-Male-Connector-DC-MagSafe-2-Cable-For-Apple-Macbook-Air-Pro-/111239821478
ebay.com.au/itm/23000mAh-Solar-Backup-Power-Bank-Charger-External-Battery-For-PC-iPhone-iPad-/371052999904

Thats the guy I got my cable from but the guy I got my same battery from has none listed currently.
Being Chinese you can't believe the mAh figure but it appears thus far offer plenty of oomph and thus bang for buck.
I'd also ignore the solar part of the equation as I believe it would take days to charge although I've not really tried.
For me its a stop gap measure should I need it which is unlikely as my bicycle and extra wheel trailer both have Son28 dynohubs which keep on top of everything enough for my use. (so far my Garmin and iPhone successfully)
I'm yet to explore trying to jury rig my E-werk to try to get a slow trickle charge into the battery.

Before anyone suggests I've insulted non Mac users - far from my intention.
There is bound to be alternatives that equal or better Macs claimed 10 hrs battery life.
I'm not stating Mac is best but merely that its up there with the best I've personally found.
Plenty of room there hopefully for someone to not feel threatened by my view that Macs are OK.
I'm merely stating an opinion that Mac products might be worth a recommendation for the OP as being close as the competition (as far as I'm aware), to the OP's parameters.
OP might have to look at secondhand though to come in within the $500 budget.
Also be aware that theres talk of iPads needing to be cellular enabled for the gps to work.

Hope some of the waffle is worthy of thought or further discussion.
If not, C'est La Vie.

I have a solar charger (5v 50,000 mAh) that will charge my tablet. It´s slow, about 4%/hour, but it DOES charge and is perfect for those days that I can´t get to an outlet. I just plug it in (tablet has to be off), and go to sleep. 8-10 hours later, the tablet is recharged. No, it´s not a 100% charge, but I don´t use 100%, so it´s a top-off. The first time I charged it, I did it from an outlet and it was quite lengthy, but now, I just put it in the sun whenever I can. The weight is about a pound, the size is roughly that of a hand calculator and you can get them really cheaply off Ebay. I´m purchasing a slightly more expensive one that will have a much faster output (probably about 12%/hour). Better still, it also charges my handlebar speakers, IPOD, and cell phone with plenty of charge leftover.

FWIW, my tablet is an ASUS 300TF. I absolutely love it...it´s my camera, phone, e-book, etc, etc.

How about laptops and the like developed for third world countries? The one laptop per child was a neat concept and some of those laptops are floating around on ebay. There must be laptop technology that has been optimized for people living off the grid in the first and third world.

Yes, and I bet it has weak processors and is very slow. People in wealthy countries want their devices to play videos.

Plug into the Mains at a Pub and have a few rounds wit the Locals