I like Seesmic. It's a flexible alternative to Tweetdeck.
If you use or are interested in hearing about Twitter, you may find this post interesting. (And you should follow me if you don't already. I promise -- my Tweets are as sharp as a butter knife -- @Cromely)
If, like many people, you want the strip off your own ears when one more person mentions Twitter, I understand. Feel free to skip this one
Once you are following more than 50 people on Twitter, the homepage just isn't useful for reading tweets. There's no sense of organization. Hence, the proliferation of free applications that make Twitter easier to use.
Tweetdeck is a popular tool because it lets you create a limited number of groups of people you follow. You can separate work people, from family, from celebrities, etc.
But Tweetdeck is missing some important features. First, it shows all the columns at once. It's great if you always use it on a large monitor, but it's more challenging on a smaller laptop screen. The ability to create different tabs for different groups would be great, but it doesn't have that.
Second, you can have only about 10 columns. If you want to add another group, or always have a search running, you have to delete a group.
Seesmic doesn't have those problems. I can have as many groups as I want. I can close a column without deleting the group. I can hide all but the one I want, or I can see as many of them as I want and scroll across the screen.
Seesmic also lets me manage multiple Twitter accounts, which is nice.
It's not perfect. Tweetdeck has a nicer color scheme. It's also easier to add users to Tweetdeck groups. Tweetdeck lets me pull up a list of everyone I follow and choose who goes in each group. On Seesmic, I have to chose a person and then assign them to all the different groups I want to put them in. Since it doesn't show a list of everyone I follow, I have to wait until a Tweet comes through from that person, and then I can assign them.
Seesmic also uses more memory. Right now, Tweetdeck is using 116 MB of memory; Seesmic is using 274 MB.
There both strong products, but the productivity advantages I get from Seesmic make it the primary Twitter application on my personal computer.