Simple interface full of deep tools, including the "Unfollow" tool to sort through people you follow on Twitter in about a dozen ways. The "Follow" tool helps you find users based on things you specify, such as "Users Who Follow @____".
Power Mode gives an exhaustive number of filters on your Twitter search (over 25 filters), such as influence, follower:following ratio, how often they tweet, keywords in bio, latest tweet, etc.
PowerPost is a beautiful tweet scheduling tool, and gives the deepest analytics I've ever seen on the short-URLs they provide.
@Inbox feature turns your mentions into an email-like interface.
You can also track tweets and keywords, click on the graph once the data's pulled up, and see the individual tweets, or tweet clusters.
Google+ Sync allows your Google+ posts to be delivered as Tweets automatically.
Also the developers are hilarious and awesome - since Twitter made the change so that you now have to manually click every action you do via 3rd-party apps (such as follow, unfollow, or block), it's gotten time-consuming to do that. ManageFlitter's team not only built a simple interface for executing hundreds of those clicks as fast as possible, they also have a video player of documentary footage, like nature or space documentaries... Classy.
I will never find a better-built, more comprehensive tool for Twitter. At least, it will take a long time.
If you have clients who want to use it for theirs, you can still add their Twitter account to your account and set it under a group (if you have a Business account), then they can log in with their Twitter account and only see that account.
Here are all the uses I have for ManageFlitter regularly:
1. Lead generation. Finding qualified leads on Twitter and contacting them.
2. Market research. Quickly see how many people on Twitter have a keyword in their bio, or in a Tweet in the last 7 days.
3. Managing my Twitter account. Following more people, unfollowing people who don't follow back.
4. Posting content and getting deep insight with PowerPost.
5. Managing replies via the @Inbox feature.
6. Searching within my followers for certain keywords via the "Search Your Account" feature.
7. Emailing reports of Twitter account stats for my clients.