The original SQLTools homepage

http://www.sqltools.net/

AskTom, you'll always find something new about Oracle and databases in general there. In addition I recommend reading his books.

http://asktom.oracle.com/

Jonathan Lewis' websites also host a wealth of information about Oracle. Author of numerous articles and recommended books.

http://www.jlcomp.demon.co.uk/
http://jonathanlewis.wordpress.com/

Wolfgang Breitling's site, always worth a visit. Lots of papers about Oracle

http://www.centrexcc.com/

HotSOS, Cary Millsap's company. Read his book "Optimizing Oracle Performance" and weep

http://www.hotsos.com/

Oracle technet, RTMF or RTFM, whatever you prefer

http://technet.oracle.com/

OraSQL, another feature rich and free Oracle frontend

http://sqlworksheet.googlepages.com/

Free TOAD, the freeware version of TOAD

http://www.toadsoft.com/lic_agree.html/

SQLDeveloper, Oracle's free java-based SQL GUI frontend

http://www.oracle.com/technology/products/database/sql_developer/index.html

TOra, which used to be one of my favorite Oracle tools in the past. Recent versions weren't that stable but try yourself

http://tora.sourceforge.net/

OraSRP, a great, free 10046 extended wait interface trace profiler

http://www.oracledba.ru/orasrp/