All of the Twitter clients, no lets be blunt, the "Social Media Management Software" below comes with a hefty price tag. While this may well put it out of range of the casual user, for business or power users, these may be a valid alternative. Price are correct at the time of writing.
Sprout Social - The application for social sprout is available free but you pay for a monthly plan to use it. Available on iOS and Android, it seems a bit limited.
socialdraft - another B2B one with a monthly fee (a large monthly fee)
MavSocial - B2B with NGO discounts, but the discounts aren't likely to be enough to attract smaller NGOs.
BufferApp - With plans from $50/month to $250, this is not likely to be popular with the casual users, and on first glance it does somewhat less than Hoostuite for the money. Included for completeness.
muFusion Social - part of the MuSigma Suite, formerly Webfluenz, this is less a social management tool than a full marketing suite.
Finally, this is not exactly a twitter client - it manages a lot more than just twitter, but it needs a mention here.
BuzzBundle is an absolutely huge marketing suite, allowing integration of multiple twitter accounts, social media accounts, creation of new accounts, and the manufacturing of public personas. You get all this for a one off price tag - or if you don't care about records and storing your past conversations, absolutely free. I haven't used it. Yet.