At work I've been using Oracle's SQL Developer lately to do my database work. It has been working great until this week when we started working on migrating to Oracle 10g. I installed the new 10g client on my workstation and while I could still use SQL Developer to connect to my old 9x schemas - trying to connect to 10g would result in a horrible crash.
After I dug around I found what seems to be the solution on the SQL Developer Forum.
Turns out there may be an issue with the JDBC drivers shipping with the latest SQL Developer. The 'fix' is to copy two .jar files from your 10g client install to the SQL Developer directories.
On my machine I copied these two files from my Oracle installation:
To SQL Developer:
Overwriting the existing files.
While this isn't an official fix - it worked for me as well as others on the forum.
FWIW - SQL Developer can also connect to MSSQL, MySQL and Access and it's free!