Big Green Eggs (or similar ceramic cookers) keep a low temp for hours. But hard to add wood chunks and you'll need a cart. I have an extra large. Great for smoking and grilling (gets really hot for searing steaks). A multifaceted cooker with a cult following (for good reason) but pricey. I did see an Egg knockoff at Academy Sports (for about $500.
A 22" Weber Kettle (Gold to catch ashes) with a device called a Smokenator (check Amazon) is hard to beat as well and much cheaper, easier to add coals and wood chunks<span style="text-decoration: underline;"></span>, easier to move, and I can keep a temp on it between 225-250 for several hours. Also great for steaks (mine has gotten up 600 degrees (you really don't need it that high--cook steaks at about 500 with a chunk of mesquite on the coals). The Weber and and the Smokenator is about 1/4 of the price of an egg with a cart and the plate setter insert.
Many like the Weber Smokey Mountain as well but I've never used one.
Years ago I used to use a Brinkmann electric water smoker which kept a temp of about 200 degrees. Hard to add wood chunks for longer smokes. But, it works and I think it's less than $100.
I've tried a multi-shelved Viking smoker as part of an outdoor kitchen a couple of times and it worked fine and you set the temp. But it's a Viking which is really a brand to impress your friends and guests. It didn't cook any better than ones $3,000 cheaper.
http://bbq.about.com/od/smokers/tp/aatp092904a.htm