VOIP on your DS: a killer app?
This software could be definitely a killer app for your Nintendo DS: SvSIP is a client for VOIP services out there using the standard protocol SIP.
I've yet to test it because I've no VOIP account using that protocol: as you know Skype uses definitely a closed protocol.
UPDATE: Now I do have a VOIP account with SIP Protocol. I opened an account on voipbuster. I tried to call my landline phone trough the Linphone software, a SIP client available for Linux (just apt-get it!). Here are the instructions about how to use linphone and voipbuster.