Twitter Advertising from the company that gave us PayPerPost
Sponsored Tweets provides a very polished UI. They seem to have taken all their learnings from PayPerPost and other Twitter advertising platforms into consideration before launching this service.
As described on Mashable,
The Sponsored Tweets platform works by giving advertisers the ability to create campaigns and select, invite, and approve Twitterers of their choosing to participate in their sponsored campaigns. On the flip side, Twitterers can set their pay rate and find opportunities to tweet on behalf of advertisers and get paid per tweet and/or click.
Sponsored Tweets used OAuth for login. You can choose to get notified about new opportunities via email or Direct Messages.