SKDR.net
This site provides is probably the net's biggest archive of Hiptop / Sidekick applications. There are many great tools to try out, and there is no better place to get them.

uClinux
Read about the progress of uClinux, a Linux variant port for the Hiptop.

Protoap
Protoap is a ringtone loader/exchanger for the Danger Hiptop or T-Mobile Sidekick. It allows users to manage their stored ringtones. It is available by personal request only.

HipTalk
HipTalk is a Jabber client for the HipTop. It allows chatting across AIM, MSN and Yahoo all through a single client.

Hipster
Hipster is a popular replacement for web browsing on the hiptop.