DCCP and SCTP are the two main alternative transport protocols on top of IP

  - UDP: unordered unreliable datagrams
  - DCCP: unordered unreliable datagrams with congestion control
  - SCTP: optionally-ordered reliable datagrams with congestion control
  - TCP: ordered reliable stream with congestion control

SCTP is the most widely used after TCP and UDP: it's often the transport used in phone networks (very popular with Ericsson); but also now with WebRTC (which uses SCTP over DTLS over UDP). The main issue being that consumer firewalls usually only support NAT for TCP, and limitedly for UDP (this is one of the 'other' reasons to look forward to IPv6)