So finding applications for Sprint's Instinct is difficult at the moment. An app store is coming, and a development contest recently ended, but so far nothing has surfaced from it.
However, there is at least one person, who goes by the name 'Killer Beagle' who is writing free apps specifically for the Instinct. I will try to keep track of useful apps here, so if there are any instinct users out there who know of apps, please post 'em!
I just downloaded KBjabber, a jabber chat client. This allows me to login to google chat and talk to people. Unfortunately, it is an unsigned application, which means that it can't run in the background, which means it can't run in the background. Still might prove useful at some point though.
To download, on your Instinct navigate to:
and enter the code: 3102881
This is known as a rumkin jump code. Rumkin allows anyone to upload an application, and it assigns a jump code for them. Rumkin then serves these files to the phone via the above URL. This provides an easy way to get the phone to install the application.