pcapsipdump is libpcap-based SIP sniffer with per-call sorting capabilities. It writes SIP/RTP sessions to disk in a same format, as "tcpdump -w", but one file per SIP session (even if there is thousands of concurrent SIP sessions).
Getting started: http://pcapsipdump.sf.net/
A secure VoIP client for Google Android based on SipDroid (http://sipdroid.org). CounterTap uses SRTP to secure the media stream. Please read the FAQs in the help forum before installing. NB: I will restart dev work later in 2010, pending resources.