The best replacement for uml is erd.

The best alternatives to uml are rated as following:
★★★★★  erd
★★★  bpmn
★★  xml
★★  sysml
★★  flowchart
★  idef
★  dfd
★  orm
★  rup
★  unified process
★  use case
★  xen
  association
  ssadm
  entity relationship
  erd when
  er diagrams

Your alternative for uml is not listed? Suggest!
Speaking about uml, one probably shouldn't forget about visio.

Other search terms related to uml and their ratings are:
★★★★★ visio
★★★ netbeans
★★ rational rose
★ rational rose 2002
You have in mind a companion for uml and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of uml in all years. Click on graph for more.
uml
erd
bpmn