Hi rmt81 and welcome to the WAB forum
As Muggsy suggests, the Brinsea range of incubators and brooders is excellent, and useful for treating a variety of wildlife casualties; their product range can be found at this link:
Brooders
Currently we use the TLC-4 brooder model, which is an excellent piece of kit for altricial nestlings, but that model has been recently discontinued and replaced by the new TLC-40. You can often find hardly-used second-hand TLC-4 units for sale at around half price on eBay though. In addition to very accurately controlling temperature, you can also maintain humidity within the chamber.
For precocial chicks or older altricial fledglings however, the EcoGlow chick brooder is very affordable and provides a safe heat (with a low 12V power step-down transformer); it can be used to provide reliable over or under heating and so is also useful for a variety of species.
For larger species, you need larger heatpads of course. Those we now prefer come from Petnap who have an eBay store at these links:
petnapltd items - Get great deals on Whelping Boxes, Whelping Kit items on eBay.co.uk Shops! http://stores.ebay.co.uk/petnapltd/M...=p4634.c0.m322
I like these because both the vinyl and metal plate options come with armoured power cables (plastic armour on the 12v supply cable for the vinyl pads, but metal armour on the mains power cable for the metal pads ... very useful if you need to keep nibbling animals (rabbits, hares, mice, etc.) safe and comfortable.
Be careful though if you opt for the mains voltage ones ... they obviously don't mix well with water, so best avoided if there is any chance of spillage from water bowls etc, and I would personally never consider using these, or indeed any electric pad, without a Residual Current Breaker (RCB) device fitted between the plug and the power socket (or on the power circuit itself).
I have found these devices to be very reliable though and so much more useful than the microwaveable pads ... though these also have their uses too, specifically to keep patients warm on the car journey to and from the vet, for instance.
In short, I don't think there is a one-fits-all solution, althought the TLC-4 comes close, and like us you probably need to equip yourself with a variety of devices.
EDIT: I should perhaps mention that apart from being a satisfied customer, I have no other connection with the suppliers mentioned, nor do I receive any incentive (I wish) for endorsement of their products.