I've been using for a couple of years now to manage multiple social media networks – multiple Twitter accounts, my Facebook page, LinkedIn and Foursquare. In addition to the multiple accounts, it allows you to schedule tweets, which I do daily for the account I run, because it gives me the opportunity to get through my feed reader in the morning, space tweets throughout the day, and be present to handle mentions and DMs without having to bounce out to my reader for more links.
They also allow you to manage teams (multiple users on one Twitter account) and give you analytics.
HootSuite went freemium in late 2010. You can still use it for free, but if you want to fully take advantage of the analytics and multiple accounts, you pay a nominal rate (about $5 a month).
I'm not nuts about ads everywhere on a blog, and I don't expect to make my living (or even a decent secondary income) from affiliating, but I'm a big fan of HootSuite, and I'm willing to recommend them without being compensated – so why not get compensated?
Check them out (they'll give you a free one-month trial), and if you like them, stick with them for a while. If you click on a link from my site, it's an affiliate link.