@ttytter, developed by Floodgap is my favourite twitter client. Despite of its minimalist look (CLI apps, text-based running on terminal), I can use it to run on many platform, including on my Palm Treo Pro. On Linux/Unix and Mac, you just need to do this:

1. download it from here
2. on the directory where you keep the download:

# mv ttytter.txt ttytter
# chmod +x ttytter

3. then you can run it by executing:

# ./ttytter
or
# ./ttytter -user=[username]:[password] -vcheck

If you are Windows user, you can do telnet/ SSH to your Linux/ UNIX machine, or if you don't have any, you can register to public access UNIX server like SDF and running ttytter from there. Same thing if you want to use your mobile phone. From my Palm Treo Pro which running Windows Mobile, I just need to install SSH client like PocketPutty, and do the rest like you do at Windows PC. That also applies to another mobile/ pda phone using any OS, as long as they have telnet/ SSH client installed.

Some simple command is:
1. type directly to post status update
2. D [username] to send direct message
3. /re [post number] to reply
4. /rt [post number] to retweet

5. /re to list @[yourusername]
6. /again to refresh your timeline
7. /dmagain to show your DM

For complete operational command, you can look here.

And for additional group support, you can take a look at my friend's blog (@vkoser) here.