For an inexpensive nailer you really can't beat a hammer.
I'm with old wiz, we need to know the application to recommend a tool. I've got a CH 18ga nailer (up to 1-3/8) and a Princess Auto combo nailer/stapler (up to 2") and they're both good for knocking together little stuff. Those slippery little nails don't hold very tight though, if it has to stay together I use glue as well and the nails are demoted to clamping duty.
I considered getting a framing nailer when I redid the deck but since we put the decking on with screws I was only putting nails in the post-and-beam undercarriage. Hammered them all in, took less time than a trip to the tool store.
A cheap framing nailer might take odd nails, watch for that.
http://www.canadiantire.ca/en/pdp/mastercraft-framing-nailer-3-1-2-in-0588434p.html#srp
--> Uses 2" - 3-1/2" 34° clipped head framing nails
A cheap roofing nailer might be heavy and might jam when you're on the roof.
Personally, I'm on the lookout for a good used pro gun, that can be rebuilt, rather than a cheap new one.