A BSD-licensed single-header-single-source OAuth1 implementation.
Use CocoaPods:
pod 'TDOAuth'
Or Carthage:
github "tweetdeck/TDOAuth"
Or just drag and drop the two sources and grab OMGUserAgent.{h,m}.
Usage is fully documented in the header.
Tested against Foursquare and Twitter. If you test it against something else, please fork and list the services you used it against here. Thanks.