PostgreSQL designing tool
There are several options in the PostgreSQL world, most of them are listed in the PostgreSQL wiki:

http://wiki.postgresql.org/wiki/GUI_Database_Design_Tools

The next version of pgAdmin will include a ER designer as well as far as I know.

PowerArchitect works with PostgreSQL, though in the free version it is somewhat limited. In general it does not support DBMS specific features (or only a very limited set).

In the commercial world, nearly all of the "big names" support PostgreSQL as far as I know (including Sybase PowerDesigner and ERwin). 

dbWrench is a not too expensive and seems to be quite nice. 

TOAD data modeler is a bit more expensive but has an impressive feature list. 

You might also want to check the options listed on this page: http://www.databaseanswers.org/modelling_tools.htm